Hyperthyroidism, or an overactive thyroid, is relatively common in older cats.
It is caused by a tumour, which in most cases is benign (non-cancerous).
Hyperthyroidism can cause a number of problems, including heart problems, so it is
essential to treat it if present.
There are a number of possible treatments, all with pros and cons.
Hyperthyroidism masks CKD.
Once the hyperthyroidism is under control, CKD may be revealed (or, if
already present, may appear to worsen) as true kidney function is revealed.
What Is the
Thyroid?
The thyroid
gland is a butterfly shaped gland, with two lobes. It is situated at the
front of the neck.
The thyroid is
responsible for the regulation of metabolism. Imbalances may sometimes
occur as follows:
hyperthyroidism
means the thyroid is overactive. This is relatively common in cats,
especially in older cats. It is thought that more than 10% of cats over the age
of ten will develop hyperthyroidism.
hypothyroidism
means the thyroid is underactive. This is very rare in cats. m
If you struggle to remember which is which, the "po" in hypothyroidism
sounds like low.
euthyroid means the thyroid is in balance.
Hyperthyroidism
(Overactive Thyroid)
What Happens in Hyperthyroidism
The most common
cause of hyperthyroidism is a tumour known as an adenoma, which is usually
benign (i.e. the tumour is not cancerous). In most cases, both lobes of
the thyroid gland are affected. The tumour usually grows over time, but
the timeframe varies from cat to cat. Because the tumour increases the
size of the thyroid gland, the gland produces more thyroid hormone than is
needed by the body, i.e. it becomes overactive, which is referred to as
hyperthyroid.
Since the
thyroid controls metabolism, this overactivity causes all of the body's processes
to speed up.
Hyperthyroidism
often causes
heart problems.
Risks associated with radioiodine therapy for feline
hyperthyroidism Advanced Veterinary Medical Imaging says "The
chronic elevation of circulating thyroid hormone levels in cats with
hyperthyroidism leads to changes in their heart muscle that predispose
them to developing heart failure." Fortunately, heart problems caused by hyperthyroidism may be reversed if
the hyperthyroidism is successfully treated, though some cats will require
ongoing treatment for heart problems.
Proteinuria may improve once the hyperthyroidism is under control.
Untreated
hyperthyroidism can also affect phosphorus and calcium levels and cause
hyperparathyroidism.
Left untreated,
hyperthyroidism will eventually be fatal. Fortunately it is treatable.
However, treating it may unmask hidden kidney disease, see
below.
Occasionally a
cat may appear to have the opposite problem to hyperthyroidism, i.e.
hypothyroidism. This means the thyroid is underactive, so metabolic
processes are slower than they should be.
Hypothyroidism
is common in dogs and humans but is very rare in cats, though recent
research indicates it may not be as rare as was previously thought. The
first cause below will not require treatment but the other two will.
If bloodtests
indicate that a cat with another disease, such as CKD, has low thyroid
levels, it is quite possible that the cat has "non-thyroidal
illness syndrome" or NTIS (formerly known as "euthyroid sick syndrome" or
ESS). The
concomitant disease.makes
T4 levels (used to diagnose thyroid problems) appear lower than they
actually are.
NTIS does not
usually need any treatment, because if you are able to bring the accompanying disease
under control, the thyroid hormones should return to a normal level.
NTIS may also be
caused by starvation, so if your cat has not been eating, this might be a
factor.
Euthyroid sick syndrome (2017) Aytug S
Medscape discusses euthyroid sick syndrome in humans but the
principles are the same.
Following Treatment for
Hyperthyroidism
Hypothyroidism
may also appear after treatment for hyperthyroidism. Often this is mild
and transient and will not need treatment, but some cats do need
temporary or permanent treatment. See below for more
information.
Spontaneous Primary Hypothroidism
For a long time
primary (i.e. not as a result of another problem) hypothyroidism was
considered extremely rare in cats. However, a recent study,
Spontaneous primary hypothyroidism in 7 adult cats
(2018) Peterson ME, Carothers MA, Gamble DA & Rishniw M Journal of
Veterinary Internal Medicine32(6) pp1864-1873, found that
seven cats examined over a 3.5 year period because of possible symptoms of
hypothyroidism did in fact have it (another
cat examined after the study period was also found to have it).
Six of the seven cats in the original study were found to have a goitre.
Four of the cats had elevated creatinine levels (2.2-3.4 mg/dl).
The cats were
treated with
100-200ug a day of levothyroxine (a thyroid hormone), as far as I can
tell only for 3-7 months (most humans with hypothyroidism remain on
levothyroxine for life), and the goitres shrank and their bloodwork
returned to normal, including their kidney values. All the cats remained
alive and well with no kidney problems some two years after the study.
It may surprise
you to learn that hyperthyroidism was
unknown in cats until 1979. Therefore there is much speculation about
possible risk factors that may increase the chances of a cat developing
the benign tumour that causes hyperthyroidism:
Chemicals known as
polybrominated diphenyl ethers (PBDEs) or organophosphate esters
(OPEs) are
commonly used as flame retardants. There appears to be a
correlation between the increased use of these products in the
home and the rise in feline hyperthyroidism:
These
products are thought to mimic thyroid hormones, and thus may be a factor
in the development of hyperthyroidism.
Flame retardant chemicals in house dust linked to
hyperthyroidism(2012) Peterson ME Insights into
Veterinary Endocrinology says "In support of that hypothesis,
major PBDE production began just before the time that hyperthyroidism was
first recognized in 1979."
Elevated PBDE levels in pet cats: sentinels for
humans? (2007) Dye JA, Venier M, Zhu L, Ward CR, Hites RA &
Birnbaum LS Environmental Scientific Technology41(18),
pp6350-56 measured levels of PBDE in 23 cats, eleven of whom had hyperthyroidism. The cats with
hyperthyroidism had three times as much PBDE in their bodies as the
healthy cats. Indoor cats had particularly high
levels, which may be because they are constantly exposed to these
compounds within their homes. Tying in with the
food study below,
indoor cats who favoured fish-flavoured canned food appeared to be most at
risk of developing hyperthyroidism; and it appears that these foods contain the
highest levels of PBDE.
The feline thyroid gland: a model for endocrine
disruption by PBDEs? (2012)
Mensching DA, Slater M, Scott JW, Ferguson DC & Beasley VR Journal of
Toxicology and Environmental Health Part A75(4) pp201-12 found
that total levels of PBDEs were not significantly different in household
cats with and without hyperthyroidism but were lower in feral cats. The
highest concentrations were found in household dust, and dust levels were much
higher in the homes of hyperthyroid cats.
Although PBDEs were phased out starting in 2004, OPEs were introduced
instead.
Silicone pet tags associate
tris(1,3-dichloro-2-isopropyl) phosphoate exposures with feline
hyperthyroidism (2019) Poutasse
CM, Herbstman JB, Peterson ME, Gordon J, Soboroff PH, Holmes D, Gonzalez
D, Tidwell LG & Anderson KA Environmental Science Technology
53(15) pp9203-9213 placed silicone collar tags on cats and checked how
much OPE the tags collected. Cats with hyperthyroidism had higher levels
of OPEs collected on their tags. Higher concentrations were also seen in
homes that used air freshener, homes built after 2005 rather than before
1989, and in cats who like to hang out on the sofa (since I sit on a sofa
to write this website, I shudder to think what my levels are like; but
hey, I don't sit on it starkers).
Dietary Concerns
Food Containers (Bisphenol A)
Epidemiologic study of relationships between
consumption of commercial canned food and risk of hyperthyroidism in cats
(2004) Edinboro CH, Scott-Moncrieff JC, Janovitz E, Thacker HL & Glickman
LT Journal of the American Veterinary Medical Association224
(6) pp 879-886 examined the records of just under 170,000 cats over a
20 year period. The study found that the consumption of canned food
appeared to carry an increased risk (almost four times the risk compared
to cats who ate dry food) of developing hyperthyroidism, and that the risk
increased for every year that the cat ate canned food.
A later study,
Risk factors for feline hyperthyroidism in the UK
(2009) Everard A, Brodbelt D, Elliott J, Syme H Journal of Small Animal
Practice50 pp406-14 found that "exposure to food packaged
in a can was identified as the major risk factor for the development of
hyperthyroidism." It also found that non-pedigree cats, indoor cats
and cats who used a litter tray were more likely to develop
hyperthyroidism.
Males appeared to be at risk only if they
ate food from pop-top cans, i.e. standard canned food did not appear to carry a
risk. However, females appeared to be at increased risk whether they
ate food from pop-top cans or other types of cans.
The reason for these findings is not known at this time, but it may
possibly be related to the lining used in these cans, bisphenol A (BPA),
which may migrate into the cans' linings.
Bisphenol A is a potential endocrine disrupter. It is structurally similar
to thyroid hormones in particular and may inhibit T3 (see below) binding
to the thyroid receptor. Human studies have shown that higher levels of
BPA in urine are associated with increased thyroid function.
Serum concentrations of bisphenol A in elderly cats and its association
with clinicopathological findings
(2021) Kovaříková S, Marsalek P, Habanova, M & Konvalinova J
Journal of Feline Medicine and Surgery23(2) pp105-114 found that there were measurable levels of BPA in the
blood of the cats in the study, and that levels were much higher in cats
who were indoors only or who were fed canned food. Levels were also higher
in mature cats (aged 7-10) but were lower in older cats).
Determination of bisphenol A in commercial cat food marketing in the
Czech Republic (2022) Maršálek P, Kovaříková S, Lueerssen F
& Večerek V Journal of Feline Medicine and Surgery24(2)
pp160-167 examined BPA levels in a number of different cat food types,
including canned, dry, foil trays and pouches. Measurable levels of BPA
were found in all the foods. Again, the canned foods contained the most
BPA, which the study attributes to "the lack of internal coating" in
pouches, trays and packets; it is thought the levels they contained may
have resulted from the manufacturing process. There was also a difference
between foods produced by different manufacturers but unfortunately the
four manufacturers are not named in the study.
Whilst these
finding are a concern, generally speaking CKD cats do better on a wet food diet (see
Which
Foods to Feed). A possible compromise might therefore be to feed food
in pouches or foil trays. If you do use cans, be sure to store any leftover
food in glass containers in the fridge rather than in the cans themselves.
The University of Warwick
has more
information on this.
Food Flavours
Evaluation of dietary and environmental risk factors
for hyperthyroidism in cats(2000)
Martin KM, Rossing MA,
Ryland LM,
DiGiacomo RF, Freitag WA
Journal of the American Veterinary Medical Association217(6)
pp853-856, found that "cats that preferred fish or liver and giblets
flavors of canned cat food had an increased risk". Fish-flavoured
foods appear to contain the highest levels of PBDE (see
above).
The classic
symptom of hyperthyroidism in cats is that the cat loses weight despite
having a good appetite and eating a lot.
Hypertension
(high blood pressure) is extremely common in hyperthyroid cats.
Hypertension in cats with chronic renal failure or
hyperthyroidism (1990)
Kobayashi DL, Peterson ME, Graves TK, Lesser M & Nichols
CE Journal of Veterinary Internal Medicine 4 pp58-62 found
that
87% of the hyperthyroid cats in the study had hypertension.
If your cat has
CKD, you may not necessarily suspect that your cat has hyperthyroidism,
because some of the
other symptoms of hyperthyroidism are also common in CKD cats, including:
increased
urination
increased
thirst
a greasy coat
spiky fur
vomiting
hoarseness
restlessness
twitching or trembling
howling
(especially at night)
a fast heart
rate (tachycardia)
sometimes a
heart murmur
scratching a lot and pulling out fur.
Newman
Veterinary has a photo of the result of such behaviour.
One major
difference is that CKD cats tend to lose weight and their appetites,
whereas hyperthyroid cats tend to lose weight despite having a good
appetite.
A small number
of
hyperthyroid
cats have a form of hyperthyroidism known as "apathetic hyperthyroidism" and become lethargic and lose their
appetite instead. This only happens in a small number (less than 5%) of
cats, but these cats are more likely to have CKD or heart disease as well
as hyperthyroidism.
PetCoach has a helpful article about
hyperthyroidism which gives the.likelihood of the various symptoms.
Masking of (Hiding)
CKD
Unfortunately,
as well has having some similar symptoms to CKD, hyperthyroidism can
actually hide the presence of CKD or make CKD appear to be less severe
than it actually is. This happens
for a number of reasons:
One of the
body's metabolic processes which is speeded up by hyperthyroidism is the
glomerular filtration rate (GFR), a measure of kidney function.
Increased GFR can lead to reduced BUN and
creatinine
levels.
Hyperthyroidism
also increases bloodflow through the kidneys, which makes them appear to
be working better.
The loss of muscle mass
commonly associated with hyperthyroidism may also reduce
creatinine
levels (since creatinine is a by-product of muscle).
Therefore it can
seem as if the kidneys are functioning well even in cases where
they are not. In other words, hyperthyroidism does not cause CKD, but it
can mask (hide) the presence of CKD, or make it look less severe than it
is.
Since over 10%
of cats over the age of ten have hyperthyroidism and around 10% of cats
over the age of ten have CKD, it is inevitable that some of these cats are
actually suffering from both conditions; but in such cats, the true
condition of the kidneys only becomes apparent once the hyperthyroidism is
treated, and it is often at this point that CKD is finally diagnosed.
This does not
mean that hyperthyroidism should not be treated; on the contrary, it is
essential that hyperthyroidism be treated. What it does mean is that you
need to do it very carefully in a cat with confirmed or suspected CKD. See
below for more information on this.
Pet Place has some information about
hyperthyroidism and kidney disease.
Diagnosis
It can sometimes be difficult to diagnose hyperthyroidism because many of
the common symptoms can be seen in other diseases (e.g. increased drinking
may be seen in cats with diabetes or CKD), and some cats have borderline
hyperthyroidism, so the usual tests may not provide a definitive
diagnosis. The vet therefore needs to look at the overall picture.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416
state "Diagnosis requires the demonstration of
persistently elevated thyroid hormone concentrations (T4, or T4 plus
fT4ed) occurring concurrently with one or more of the typical clinical
signs."
The guidelines for diagnosis and treatment may be summarised as follows:
Category
Symptoms Present
T4
Thyroid Gland
Suggested Response
Group 1:
Classical clinical disease
Yes
Elevated
Enlarged
Treat for hyperthyroidism.
Group 2:
Possible hyperthyroidism
with probable non-thyroidal disease (e.g. CKD)
Yes
Normal
Repeat T4 test, together with fT4ed
test, after 2–4 weeks.
Evaluate for other diseases.
Consider T3 suppression or thyroid scintigraphy
Group 3:
Enlarged thyroid without clinical
hyperthyroidism
No
Normal
Enlarged
Monitor symptoms.
Repeat T4 test
in 6 months
Group 4:
Subclinical hyperthyroidism
No obvious clinical signs but some
symptoms suggestive of
hyperthyroidism
Elevated
Repeat T4 test in 2 weeks, treat if
elevated.
If normal, repeat T4 test in 6 months
Group 5:
Clinical hyperthyroidism with confirmed
non-thyroidal illness
Yes
Elevated
Treat for hyperthyroidism.
Manage other diseases as appropriate.
Group 6:
Clinically normal
No
Elevated
Repeat T4 test.
Treat if elevated.
If normal, monitor symptoms and repeat T4 test in 6 months
Here is some information on the tests mentioned in the table which are
used to diagnose hyperthyroidism:
The vet may
also detect high blood pressure and/or a faster heart rate than usual,
while the heart may be enlarged.
One vet told me
he has found that if he pinches a cat's ear, its pulse
will slow down, except in hyperthyroid cats. Obviously,
this is not a proper method of diagnosis, but you may want to check this
occasionally at home.
Blood Tests
Blood Chemistry and Complete Blood Count
Apart from
specific blood tests for hyperthyroidism (see below), routine blood
tests are usually run, which may show the following:
Short-term biological varioation of serum thyroid hormone concentrations in
clinically healthy cats
(2020) Prieto JM, Carney PC, Miller ML, Rishniw M, Randolph JF, Lamb SV,
Place NJ & Peterson ME Domestic Animal Endocrinology71 epub
106389 found that there are large variations in thyroid hormone levels in
healthy cats, but there is less variation within an individual cat. The
study concludes "This suggests that for all serum thyroid hormones, but
especially serum TSH and T3 concentrations, comparing individual cat's
hormone results to a population-based reference interval may be misleading,
especially in those with early or subclinical thyroid disease. Clinicians
might improve the diagnosis of feline thyroid disease by establishing
baseline concentrations of T4, free T4, T3, and TSH for individual cats
(ideally when healthy) and applying reference change values to subsequent
measurements."
Using patient established reference intervals to
diagnose thyroid disease in cats
(2020) Singer L Clinician's BriefApr 2020 pp58-59 explains
more about setting reference intervals for individual cats.
T4 or Total T4
The most widely
used blood test for thyroid function in cats is the T4 test, also known as
Total T4. This measures levels of a hormone called thyroxine.
What is T4? How is the T4 test used to diagnose
hyperthyroidism in my cat? (2010)
Peterson ME
Insights into
Veterinary Endocrinology
explains more about T4.
Some vets can
run this test in house, but many prefer to have it run by an external
laboratory.
Comparison of in-clinic point-of-care and reference
laboratory total thyroxine immunoassays for diagnosis and post-treatment
monitoring of hyperthyroid cats
(2018) Peterson ME, Rishniw M, Bilbrough GE & Cote KB Journal of Feline
Medicine & Surgery20(4) pp319-324 compared the
IDEXX Laboratories Catalyst Total T4 in-house test with
one performed at a laboratory and found that "the in house analysis of TT4
provided reliable and accurate values for TT4, comparable to the reference
lab. It appears that the in house testing is largely equivalent to the
reference lab for classification of cats as hypo- eu- or hyper- thyroid,
however borderline cases may need to be re-tested or confirmed by other
methods."
Here is a rough
guide to T4 levels but the laboratory used by your vet will provide its
own reference range, which you should rely on:
A T4 reading over around 5 ug/dl or 64 nmol/L indicates that hyperthyroidism is probably present, but
this must be
considered in conjunction with other findings, because in a small number
of cats (less than 10%), the T4 can be elevated even if the cat does not
have hyperthyroidism.
What steps
should you take if your cat's T4 level is elevated? According to
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416:
If T4 is
elevated, the thyroid is enlarged, and there are clinical signs of
hyperthyroidism, you should treat for hyperthyroidism.
If T4 is
elevated, there are clinical signs of hyperthyroidism and your cat has
another disease (e.g. CKD), you should treat for hyperthyroidism and also
treat the other disease. See
below for more information on managing both
hyperthyroidism and CKD.
If T4 is elevated but there are no real signs of hyperthyroidism and no
enlarged thyroid, you
should repeat the T4 test in 2 weeks, and treat for
hyperthyroidism if
it remains elevated. If, however, the second T4 test is normal, you
should repeat the T4 test in 6 months.
T4: Grey Zone
The table above
contains a grey zone area, which as you can see is still within the normal
range, but cats with T4 in the grey zone need to be looked at more closely. This is because the normal ranges do not apply
to older cats (over the age of 10), so older cats with a T4 reading in the
grey zone may nevertheless have hyperthyroidism. A grey zone reading may also not be accurate
in cats with other illnesses, such as CKD.
Michigan State University Veterinary Diagnostic
Laboratory
says "Most
hyperthyroid cats have high serum concentrations of thyroid hormones.
However, there are a small proportion of hyperthyroid cats which have
normal or only slightly elevated thyroid hormone concentrations. This
phenomenon is termed "occult hyperthyroidism." A number of possible
explanations exist for this phenomenon including the presence of thyroid
pathology which is not yet sufficiently severe to be easily documented
(mild disease); the presence of a concurrent non-thyroidal illness; and
day-to-day or week-to-week fluctuations in thyroid hormone concentrations
in hyperthyroid cats which sometimes dip into the reference range."
In fact, about
10% of cats in the grey zone will have hyperthyroidism despite having a
theoretically normal T4 level.
Evaluating another thyroid function test for
hyperthyroidism in cats
(2015) EveryCat (formerly Winn Feline Foundation) says "Approximately 10% or less of hyperthyroid cats, and over 30% of
cases with a mild or early case of hyperthyroidism, have a T4
concentration remaining in a normal reference range. In addition,
cats with concurrent nonthyroidal illness can have suppressed high
serum T4 concentrations to within the normal reference range."
One such
concurrent illness is CKD.
Diagnosis of hyperthyroidism in cats with mild
chronic kidney disease (2008)
Wakeling J, Moore K, Elliott J & Syme H Journal of Small Animal
Practice49(6) pp287-94 found that the cats in the study with
both CKD and signs consistent with hyperthyroidism nevertheless had normal
T4 levels.
Be sure your
vet allows for this when interpreting test results, especially if your cat
already has CKD.
Cats in the
grey
zone may or may not need treatment, depending upon whether symptoms are present.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416
advise:
If T4 is
normal but there are clinical signs of hyperthyroidism, you should repeat
the T4 test and also perform a
free fT4 test after 2–4 weeks. You should also consider
T3 suppression or
scintigraphy tests, and
evaluate the cat for other diseases that may be causing some of the
symptoms (e.g. CKD).
If T4 is normal
and there are no clinical signs of hyperthyroidism, but the thyroid gland
is enlarged, you should monitor the cat's symptoms and repeat the T4
test in six months.
T4: Low
A T4 level
below normal,
indicating hypothyroidism or an underactive thyroid, is extremely rare in cats.
There are usually two main reasons for this:
Cats who are receiving treatment for hyperthyroidism may have been
overtreated, so the opposite problem of an underactive thyroid occurs.
This may happen after I-131
treatment in particular. Often this is mild and transient and will not
need treatment, but some cats do need treatment, see
below.
Cats with a low
T4 level who also have another illness such as CKD may have
Non-Thyroidal Illness Syndrome
(though these cats tend to have T4 in the low normal range rather than in
the below normal range).
Free T4
by Equilibrium Dialysis
Occasionally
both T4 and T3 will appear normal,
but you still suspect your cat may have
hyperthyroidism.
This happened to us with Ollie (who did not have any other diseases at the
time): he had so many symptoms of hyperthyroidism
but his T4 and T3 tests came back as completely normal.
In such cases
you should ask your vet for a free T4 by equilibrium dialysis
test. This test measures the amount of T4 available for use in the body. There is little point in running this test if your cat is firmly in
the normal T4 range, or if your cat is in the likely hyperthyroidism
range, but it can be helpful with borderline cases.
This test
should not be relied upon in isolation (it should be run in conjunction
with T4 tests), but it may assist with making a diagnosis.
Confirming the diagnosis of hyperthyroidism: the T3
suppression test (2011) Peterson ME Insights into
Veterinary Endocrinology says "In the majority of these
hyperthyroid cats in which a normal total T4 concentration is found,
simply repeating the total T4 analysis with simultaneous measurement of
free T4, as well as ruling out any concurrent disease, will easily confirm
the diagnosis."
Antech
Diagnostics stated (2008) "The combined measurement of T4 over 2.3 ng/dL
[over
30 nmol/L in international values] and FT4 over 40 mmol/L or with TSH
under 0.03 ng/mL is of diagnostic value for the confirmation of
hyperthyroidism in any cat with mild or previously diagnosed chronic
kidney disease."
Around 70% of
cats in the grey zone for T4 but with a high
free T4 level will in fact
have hyperthyroidism.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416 say
"A T4 value in the upper half of the reference interval combined with an
elevated fT4ed supports a diagnosis of hyperthyroidism."
Ollie's free T4
test came back indicating that he did in fact have hyperthyroidism. Once
he was placed on medication, he improved greatly.
You may be
wondering why the free T4 test is not run instead of the T4 test.
What is T4? How is the T4 test used to diagnose
hyperthyroidism in my cat? (2010)
Peterson ME Insights into
Veterinary Endocrinology says "the
free T4 test is a more sensitive diagnostic test than is the T4 test,
i.e., the free T4 test will be diagnostic in 98.5% of hyperthyroid cats
whereas the total T4 test will be diagnostic in 91% of cats with the
disease. However, many cats suffering from other illnesses NOT associated
with hyperthyroidism will have false-positive
results with the free T4 test. For that reason, the total T4
test remains the diagnostic test of choice for cats with suspected
hyperthyroidism because we almost never see false-positive results with
the total T4 test."
What if the
free T4 level is normal?
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416 say
"A normal free
T4 makes the diagnosis of hyperT unlikely, but if a strong clinical
suspicion remains, repeat thyroid testing in 6 to 8 weeks or a technesium
scan is indicated."
Some vets also run a T3 suppression test, which looks at levels of another hormone, T3 (tri-iodothyronine). Cats with
hyperthyroidism usually have elevated T3 levels.
This test seems to be falling out of favour.
Confirming the diagnosis of hyperthyroidism: the T3
suppression test (2011) Peterson ME Insights into
Veterinary Endocrinology discusses the T3 suppression test but
points out its disadvantages include the test being lengthy (it is run
over three days, with the caregiver having to pill the cat every eight
hours for two days).
TSH (Thyroid Stimulating
Hormone)
Another
test that may be used when the T4 results are not clearcut is the TSH
test. This test is commonly used in humans to check for thyroid
problems, but was not commonly used in cats because no assay had been
developed for use in cats. Recently it appears that canine TSH tests
may be applicable to cats, so some vets now run this test. This test
cannot be used on its own, but may be of some use when used in
addition to other tests.
Cats with
hyperthyroidism would be expected to have very low levels of TSH, whereas
healthy cats should have measurable levels.
Evaluation of serum thyroid-stimulating hormone
concentration as a diagnostic test for hyperthyroidism in cats
(2015) Peterson ME, Guterl JN, Nichols R & Rishniw M Journal of
Veterinary Internal Medicine29(5) pp1327-34 measured TSH in
both healthy cats and hyperthyroid cats. The study found that 69.9% of
cats without thyroid problems had measurable levels of TSH, but so did 2%
of the cats with hyperthyroidism. Conversely, 98% of hyperthyroid cats had
undetectable levels of TSH, but so did 30% of older healthy cats. The
study concludes "Measurement
of serum TSH represents a highly sensitive but poorly specific test for
diagnosis of hyperthyroidism and is best measured in combination with T4
and fT4."
That is to say, if a cat has a high T4 or free T4 level and undetectable
levels of TSH, this would support the likelihood of hyperthyroidism,
whereas if a cat has T4 or free T4 in the grey zone but has measurable
levels of TSH, hyperthyroidism is less likely.
The TSH test is
sometimes used in cats who have developed hypothyroidism following
treatment for hyperthyroidism (see
below).
Thyroid Scintigraphy (Technetium Scan)
Some centres offer scintigraphy to diagnose hyperthyroidism. This is
often offered prior to I-131 treatment so the treatment can be tailored to
the cat's needs, but may also be offered if there is some debate over
whether your cat has hyperthyroidism or not.
During the scan, a radioactive solution (technetium) is injected into a
vein and is taken up by the thyroid. Apart from confirming the presence of
hyperthyroidism, this test also allows the precise areas of concern to be
seen.
The scan only takes about 20-30 minutes, but because radioactive substances
are used, the cat usually needs to be
kept in the hospital for 24 hours after the scan.
The main downsides of thyroid scintigraphy are limited
availability (it is usually only available at centres which perform I-131
treatment) and cost (it seems to cost at least US$500).
There are a
number of different treatments available, all with pros and cons. This
section describes the available treatments, the risks of these treatments
and the precautions which should be taken to reduce those risks.
It is essential
to treat hyperthyroidism because it is such a strain on the cat's body,
particularly the heart.
Effects of feline hyperthyroidism on kidney function: a review
(2016) Vaske HH, Schermerhorn T & Grauer GF Journal of Feline Medicine
& Surgery18(2) pp55-9 states "Untreated, hyperthyroidism
is eventually fatal."
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416 say
"Hyperthyroidism in cats is a life-threatening disease requiring prompt
veterinary attention."
In addition,
Feline hyperthyroidism and its relation with renal
function (2006) Daminet S
Presentation to the 31st World Small Animal Veterinary Association World
Congress says "The presence of a hyperthyroid state
could contribute to the development or progression of CRF."
Treatment
Risks: Unmasking CKD
Treating
hyperthyroidism is usually safe and effective. The main concern is that
doing so it may unmask previously hidden CKD (see
above) or may worsen
existing CKD (though what is actually happening is
that the true degree of renal function is becoming apparent). This is a very
real concern:
Hyperthyroidism and the kidney (2006)
Langston CE & Reine NJ Clinical Techniques in Small Animal Practice21(1) pp17-21 says "overt renal failure occurs in approximately 30% of
cats treated for hyperthyroidism."
Unfortunately it is difficult to
predict which cats will go on to exhibit kidney problems after treatment
for hyperthyroidism.
An investigation of predictors of renal
insufficiency following treatment of hyperthyroidism in cats
(2008) Riensche MR, Graves TK, Schaeffer DJ Journal of Feline
Medicine & Surgery10(2) pp160-6 states that "our study
suggests that the results of routine pre-treatment clinical data cannot
be used to reliably predict renal function after treatment for
hyperthyroidism."
A cat treated for hyperthyroidism may also develop the opposite problem
of hypothyroidism, an underactive thyroid. Hypothyroidism can damage the
kidneys further so it is essential to try to reduce the risk of this
happening.
Because of
these concerns, some vets may decline to treat hyperthyroidism in CKD cats, or may
undertreat the condition. This is not recommended because of the risks of
leaving hyperthyroidism untreated (see
above).
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416 state
"The Panel recommends treatment of hyperthyroid patients regardless
of concurrent disease – including cats with CKD."
In any event, declining to treat will not protect the
kidneys. The guidelines go on to say:
"Keeping cats with azotemia [elevated kidney values] ‘a little bit’ hyperthyroid to
increase renal perfusion and lower creatinine levels is deleterious. This
approach can exacerbate renal damage while giving a false sense of
security based on an artificially lowered creatinine level. Elevated T4
causes increased beta-adrenergic activity and activation of the
renin–angiotensin–aldosterone system, leading to increased cardiac output,
volume overload, sodium retention, renal hypertension and glomerular
sclerosis, ultimately progressing to, or worsening, CKD."
Therefore you
should treat hyperthyroidism but it is wise to start with a conservative
approach (see below).
Effects
of methimazole on renal function in cats with hyperthyroidism(2000) Becker TJ, Graves TK, Kruger JM,
Braselton WE and Nachreiner RF Journal of the American Animal Hospital
Association36 pp215-233 discusses the use of methimazole (a
medication for hyperthyroidism in cats) and its effects on unmasking CKD.
Total T4 should always be in the normal range, but whereabouts within that
range depends upon whether the cat has CKD or not.
Treatment
Goals: Non-CKD
When treating
feline hyperthyroidism in otherwise healthy cats, the target total T4 should be in the lower half of
the reference interval.
Generally speaking, the goal is a T4 in the low 2s
μg/dl (USA) or around 25-30 nmol/L (international).
How to dose and monitor hyperthyroid cats on
methimazole (2012) Peterson ME
Insights into Veterinary Endocrinology
says of healthy cats "It is important to keep the serum T4 concentration within the
mid-normal range and not have even mildly high or high-normal values on
methimazole. For example, if the T4 reference range is listed as 0.8-4.0
μg/dl (10-50 nmol/L), my goal is maintain the T4 values between 1.5-2.5
μg/dl (20-32 nmol/L)."
Treatment
Goals: CKD
For CKD cats, Dr Peterson advises a somewhat higher target range.
Unmasking kidney disease in hyperthyroidism cats
after treatment (2014) Peterson
ME Insights into
Veterinary Endocrinology states "my
"goal" in treating cats with hyperthyroidism is to reduce the total T4
concentration into the middle of the reference range (e.g., 2.0-3.0 µg/dl
with your lab)." The reference range for the laboratory in question was 1.0-4.0 µg/dl.
I would not let it fall below 1.5
μg/dl (20 umol/L) in a CKD cat.
Treatment Choices
All the
available treatments are effective, even though some of them only control
the hyperthyroidism rather than cure it. The costs are broadly similar
too, assuming your cat lives for more than two years with hyperthyroidism.
The treatments all carry similar risks:
Effects of feline hyperthyroidism on kidney
function: a review (2016) Vaske HH, Schermerhorn T & Grauer GF
Journal of Feline Medicine & Surgery18(2) pp55-9 says
"Regardless of the treatment modality used, when hyperthyroidism is
successfully treated, there will be a decrease in renal excretory
function, a decrease in the GFR, and an increase in serum creatinine."
Feline hyperthyroidism and its relation with renal
function (2006) Daminet S
Presentation to the 31st World Small Animal Veterinary Association World
Congress has a table summarising the results of treating
hyperthyroidism in a
number of different studies in cats using the usual treatments.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416
outlines the pros and cons of the various treatments.
Non-Curative Treatments
These
treatments will control the hyperthyroidism, and are often very effective,
but they will not cure it.
The main
advantage of these treatments is that you have room for manoeuvre if your
cat develops CKD or if existing CKD worsens after treatment, because an
adjustment of the dose is possible, which is not the case with the more
permanent forms of treatment.
Hill's
introduced a new food called Hill's Prescription Diet y/d in 2011. It is available
in both
canned and
dry formulas.
The food
contains reduced levels of iodine. Iodine is necessary for the production
of the thyroid hormones (T4 and T3), so the reasoning is that reduced
levels of iodine mean reduced levels of thyroid hormones and thus the
hyperthyroidism can be controlled.
Hill's claims
that feeding the food exclusively for three weeks can control
hyperthyroidism.
Effects of an iodine-restricted food in client-owned
cats with hyperthyroidism
(20140 van der Kooij M, Bečvářová I, Meyer HP, Teske E & Kooistra HS
Journal of Feline Medicine and Surgery16(6) pp491-8 found that
symptoms and thyroid blood values improved in hyperthyroid cats after four
weeks of eating this food, with no side effects.
Effect of feeding an iodine-restricted diet in cats
with spontaneous hyperthyroidism (2015) Hui TY, Bruyette, DS,
Moore GE & Scott-Moncrieff JC Journal of Veterinary Internal Medicine
29(4) pp1063-1068 also looked into how effective this diet was. It found
that 41.66% of the cats had normal thyroid values 21-60 days after
beginning the diet, and after 61-180 days 83% of cats had thyroid values
in the normal range. However, symptoms such as fast heart rate did not
improve. The study concludes
"Restricted-iodine
diets were effective at maintaining serum TT4 concentrations within
reference ranges for a majority of cats with spontaneous hyperthyroidism
over 1 year, although not all clinical signs of hyperthyroidism improved."
Hill's states
that a cat with hyperthyroidism can eat this food only and nothing else.
and that
this food should not be fed to other cats in the household. However,
Comparison of health parameters in normal cats fed a
limited iodine prescription food vs a conventional diet
(2018) Paetau-Robinson I, Melendez LD, Forrester SD, Armbrust LJ,
Refsal KR & Burris PA Journal of Feline Medicine and Surgery
20(2) pp142-148 fed y/d to healthy cats for two years and the cats did
not develop hypothyroidism or exhibit a change in thyroid gland height.
Whilst I would not feed this food exclusively to healthy cats, it may be
acceptable to leave this food out only for all your cats while you are out
of the home if your vet agrees.
The food does appear to do what it is supposed to, but will cease to have
an effect if you do not feed it exclusively. Since hyperthyroidism is a
progressive disease, it is possible that over time the food alone will not
be sufficient to control your cat's hyperthyroidism.
Healthy
cats with normal thyroid function should not be allowed to eat the food
because it could cause problems.
Hill's y/d
contains the following levels of phosphorus and protein, which are
within the usual guidelines for CKD cats:
Variety
Phosphorus %
(Dry Matter Analysis)
Protein %
(Dry Matter Analysis)
Hill's y/d canned
0.65
33.70
Hill's y/d dry
0.65
36.30
Why/d?
Peterson ME Animal Endocrine Clinic gives Dr Peterson's opinion of
Hill's y/d.
Medications: Methimazole and Carbimazole
Medications may
be used used to regulate the production of thyroid hormone. The two main
treatments are methimazole or carbimazole. In the USA carbimazole is not
currently available so you will be offered methimazole.
Antithyroid drug treatment for hyperthyroidism:
brand name, generic or compounded drug?
(2012) Peterson ME
Insights into Veterinary Endocrinology gives an overview of the various drugs used for
hyperthyroidism in cats and the forms in which they are available.
Medications are a relatively simple way of treating hyperthyroidism and
are very effective but they can have side effects, and they only control
the condition, they will not cure it. Therefore if you stop the treatment,
your cat's hyperthyroidism will return. In addition, since hyperthyroidism
is a progressive disease, you will probably find that you will need to
increase the dose over time.
When
handling any form of thyroid medication, you should wear gloves in
order to avoid absorbing any of the treatment yourself.
Veterinary Information
Network
has some information on this.
Methimazole is available in pill, oral suspension and transdermal (ointment)
form in the USA. The trade
name for the feline pill version of methimazole is Felimazole but you might
possibly be offered the human version, known as Tapazole.
Methimazole Transdermal
In the USA you also have the option of transdermal methimazole from
compounding pharmacies. Transdermal medications are usually applied to the
ears.
Efficacy and safety of transdermal methimazole in the
treatment of cats with hyperthyroidism
(2004) Sartor LL, Trepanier LA, Kroll MM, Rodan I, Challoner LJ Journal of Veterinary
Internal Medicine18(5) pp651-5
found that transdermal methimazole took longer to work
than the oral version but appeared to have fewer side effects, so this might
be one option if your cat does not tolerate oral methimazole very well.
However, bear in mind that
Transdermal application of methimazole in hyperthyroid
cats: a long-term follow-up study (2014) Boretti FS,
Sieber-Ruckstuhl NS, Schäfer S, Gerber B, Baumgartner C, Riond B,
Hofmann-Lehmann R & Reusch CE Journal of Feline Medicine and Surgery 16(6) pp453-9 found that transdermal methimazole is safe but that it
can be difficult to keep the T4 levels consistently within range over the
longer term.
Felimazole in pill form is also available in the UK. In 2017 a new
liquid form was introduced called Thyronorm. It is available in 30ml
and 100ml bottles with an in-use shelf life of six months. Thyronorm
is flavoured "with added honey for even greater pet acceptance", which
mystifies me because I don't know any cat who likes honey (cats lack
the taste receptors for sweet). It also contains benzoic acid,
which cats lack the pathways to metabolise (see
Holistic Treatments for more on benzoic acid).
Methimazole Dosingin Non-CKD Cats
I sometimes hear from people whose vets have them begin with a dose of 5
mg once or even twice a day. This may become more common in the UK because
the recommended starting dose for Thyronorm is "5mg active ingredient per
day, ideally split into two doses." This size
of starting dose makes me nervous, and some cats on this dosing regimen
can
become quite ill, particularly if they had CKD before they began treatment
for hyperthyroidism (though non-CKD cats may also be affected). Whilst these dosages are not excessive per se,
it is much safer to start at a lower dose and build up gradually to the
dose your cat requires, so the cat's body has time to adapt.
Diagnostic and treatment options for feline
hyperthyroidism (2003), Nelson R Presentation to theWorld Small
Animal Veterinary Association World Congress
recommends starting at a dose of 2.5mg once a day for the first two
weeks, increasing to 2.5mg twice a day if the cat is showing no
adverse reaction and tests indicate that a higher dose is required (these doses are per cat, not per pound or kg).
Later guidelines,
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416,
suggest a lower dose may also be suitable,
stating "Methimazole should be started at a dose of 1.25–2.5
mg per cat twice daily (q12h). Twice daily dosing is associated
with less serious side effects than a higher dose once daily (q24h). After
the cat becomes euthyroid with q12h dosing, giving the total daily dose
q24h may maintain euthyroidism and increase owner compliance. Transdermal
methimazole preparations, when available, can be useful for uncooperative
cats. In such cases, the same or a slightly higher starting dose than for
the oral route should be used."
The guidelines go on to say "Most hyperthyroid cats are euthyroid within 2–3 weeks of commencing
treatment. If the cat is still hyperthyroid, methimazole dose adjustments can be made in
increments of 1.25–2.5mg/day until euthyroidism is achieved."
If the cat's T4 falls too low, the guidelines advise "If T4 drops below the lower end of the reference interval, the
methimazole dosage should be reduced in decrements of 1.25–2.5 mg/day
and the T4 and renal parameters rechecked in 1 week."
How to dose and monitor hyperthyroid cats on
methimazole (2012) Peterson ME
Insights into Veterinary Endocrinology
explains more about dosing and monitoring cats on methimazole. He says
"It is important to keep the serum T4 concentration within the mid-normal
range and not have even mildly high or high-normal values on methimazole.
For example, if the T4 reference range is listed as 0.8-4.0 ug/dl (10-50
nmol/L), my goal is maintain the T4 values between 1.5-2.5 ug/dl (20-32
nmol/L)."
Methimazole Dosing in CKD Cats
For cats with pre-existing CKD, a lower starting dose is wise.
How to treat a cat with hyperthyroidism and
concurrent renal insufficiency (2010) Daminet S
Presentation to the 35th World Small Animal Veterinary Association World
Congress states "methimazole or carbimazole are used
(orally or transdermal), at a low starting dose (i.e. 1.25 mg orally once
a day). This allows assessing the impact of anti-thyroid therapy on renal
function. These patients should be monitored every 2 weeks. Dosage
adjustments should be made prudently".
Methimazole Side Effects and Overdosing
Side effects include lack of appetite, vomiting and in rare cases
anaemia. Cats on methimazole may
also have elevated
liver enzymes and low
white blood cells.
Veterinary
Partner discusses possible
side effects, including itching, particularly on the face.
Fortunately side effects usually do resolve.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416
state "Most side effects appear within the first 4–6 weeks of therapy and
are less common after 2 or 3 months of treatment."
In rare cases in humans, methimazole has been linked to acute
pancreatitis.
Acute pancreatitis induced by methimazole therapy
(2012) Abraham A, Raghavan P, Patel R, Rajan D, Singh J & Mustacchiaa P
Case Reports in Gastroenterology6(2) pp223–231 reports on one
case, but other cases have been reported more recently according to a
report from a 2019 conference,
Findings confirm link between methimazole and risk
for acute pancreatitis (2019) Oakes K. Human patients in Europe
who have previously had acute pancreatitis are therefore advised not to
take methimazole. If your cat is prone to pancreatitis, you might wish to
discuss alternative treatments with your vet.
If your cat is on methimazole and you also wish to use
mirtazapine
(an appetite stimulant), I would talk to your vet
about using a reduced dose of mirtazapine. This is because I have heard from a couple of
people whose cats were on treatment for hyperthyroidism who did not seem to
do well on mirtazapine. This is probably because methimazole reduces levels of a liverenzyme
called CYP2D6 which helps to
clear mirtazapine from the body.The
University of Maryland Medical Center used to discuss this but I can't
find any links about this at present.
Unfortunately, as
Drs Foster & Smith
say, "Signs of toxicity or overdose are similar to the side
effects listed; liver and blood disorders are the most common."
Carbimazole
Carbimazole is another form of thyroid medication, which is commonly
prescribed in the UK but only rarely used in the USA.
Carbimazole is actually converted into methimazole in the cat's body, but
it tends to have fewer side effects, probably because carbimazole appears
to act more slowly than methimazole and is absorbed less quickly. I have
heard of quite a few cats who did badly on methimazole but who could
tolerate carbimazole just fine, so if this applies to you,
you may wish to try carbimazole instead and see if your cat does better on
it.
Newman
Veterinary mentions that there are fewer side effects with
carbimazole than with methimazole.
Generic carbimazole used to be the drug treatment of choice for
hyperthyroid cats in the
UK, but once veterinary formulations of thyroid medications became
available (which you will not be surprised to hear are much more expensive
than generic carbimazole), vets became legally obliged to prescribe these first.
This is because UK regulations require that a medication approved for a
species in the UK must be used as the first choice, and alternatives can
only be considered if the first choice medication does not work or causes
too many side effects. This policy is known as the cascade and
The British Small Animal Veterinary Association
explains more about how it works.
Therefore you will most probably be offered either the veterinary version of carbimazole called
Vidalta, or methimazole with the brand name of Felimazole
or Thyronorm.
Personally I would always opt to use carbimazole over methimazole.
Vidalta is only available in 10mg or 15mg tablets which are slow
release, so they should not be crushed. Some people were concerned about how
to use Vidalta in cats because of this, bearing in mind that many cats
need a relatively small dose. However, in
How to dose and monitor hyperthyroid cats on
methimazole (2012) Peterson ME
Insights into Veterinary Endocrinology
mentions that "it is
estimated that 15 mg of this preparation is equivalent to approximately
7.5 mg of conventional methimazole",
but that absorption is higher if given with food.
If you have any problems with the branded products, your vet can still get the generic carbimazole for you,
and it is available in a 5mg size. I used generic carbimazole
for my Ollie simply because he tolerated it better (as a bonus, it saved
me money). See below for dosing.
Carbimazole Dosing
Dosing is similar
to that for methimazole,
although carbimazole may be given up to three times a day if necessary
(i.e. the total daily dose can be divided into three).
These
treatments will cure the hyperthyroidism, and are usually very effective.
The main
disadvantage of these treatments is that you have less room for manoeuvre
if your cat develops CKD or if existing CKD worsens after treatment,
because an adjustment of the dose is not possible as it is with medication
or diet.
Some vets
recommend a medication trial before opting for curative treatment.
This means that the hyperthyroidism is treated initially with medication
(methimazole or carbimazole). This enables you to adjust the dose
should you see any signs of problems in an attempt to balance the conflicting needs of the kidneys and the
thyroid, and ideally avoid a
crash; whereas
this is not possible with the curative
treatments, where the treatment is complete and no finetuning is
possible.
Feline hyperthyroidism
(2009) Graves TK CVC in Washington DC Proceedings explains more about using medication as the first
choice treatment and recommends doing this for at least thirty days.
says "It seems
reasonable (although not evidence-based) to recommend a trial therapy in
any hyperthyroid cat presented with one or more of the following: BUN or
serum creatinine values at the high end of the reference interval, a low
USG, an increased UPC or marked ultrasonographic kidney abnormalities.
Currently the most useful predictive parameter seems to be GFR
measurement. A low pre-treatment GFR was predictive of the development of
CRF in several studies. However, measurement of GFR is often impractical
in a clinical setting."
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416
differentiate between cats depending upon their stage of CKD, saying "Manage
IRIS stage 1 and 2 cases as though they are non-azotemic. If the patient
responds favorably and renal function is stable using a reversible
treatment, then consider an irreversible FHT treatment. IRIS stage 3 and 4
patients warrant a more prudent approach consistent with
Group 5 status; for
example, using lower doses of methimazole and more aggressive management
of CKD. If a permanent treatment for FHT is pursued, careful monitoring
and aggressive kidney support may be required during the period of
regeneration of previously suppressed normal thyroid tissue."
Such a trial
may also be wise in cats with some other conditions.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416
say "Hyperthyroid
cats at increased risk of complications, including those with
cardiovascular disease or severe hyperthyroidism, may benefit from
treatment with methimazole or beta-adrenergic antagonists before
definitive treatment with radioactive iodine or surgery."
Personally, I
would want to do a trial in any of my cats.
Surgery (Thyroidectomy)
Surgical removal of the thyroid gland, or
thyroidectomy, is a possible treatment option for hyperthyroidism. Although this is usually successful, it is of course
invasive, exposing the cat to the risks of
anaesthesia.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416
state "Surgery and anesthesia are sometimes
associated with substantial procedural morbidity and mortality.
Hypocalcemia occurs in a widely varying range (6–82%) of thyroidectomy
patients, depending on the surgical method chosen."
There is also a risk that too much of the gland may be removed, resulting in the
opposite problem, an underactive thyroid (hypothyroidism).
Complications of thyroidectomy in cats:
post-operative hypothyroidism (2012) Peterson ME
Insights into Veterinary Endocrinology says "Most
hyperthyroid cats are readily cured quite easily with the use of surgical
thyroidectomy" but goes on to say that most of them will develop
hypothyroidism, which may be temporary or permanent.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416
nevertheless state "Surgical thyroidectomy is associated with a high rate of both short- and
long-term success, with most studies showing >90% of cats achieving
euthyroidism postoperatively, with a relapse rate approaching 5% within
3 years."
I hear from very few people whose cats undergo this treatment these days,
and it would not be my first choice.
Radioactive Iodine Treatment (I-131)
Radioactive iodine treatment (I-131) is considered by some to be the "gold
standard" treatment for hyperthyroidism.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney HC,
Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR
Journal of Feline Medicine and Surgery18(5) pp400-416 say
"Experts generally agree that radioiodine is the treatment of choice for
most cats with FHT."
The treatment entails the cat being given a dose of radioactive iodine,
which destroys the diseased thyroid tissue while leaving healthy thyroid
tissue intact. The treatment is very effective:
Risks associated with radioiodine therapy for feline
hyperthyroidism Advanced
Veterinary Medical Imaging states 'The overwhelming majority of cats
(>95%) are cured by a single therapy with only a small percentage of cats
requiring additional radioiodine therapy (4%) or becoming persistently
hypothyroid (1%)."
The treatment has no major side effects, but it is only available in a
limited number of facilities, and the cat has to be hospitalised for a
period ranging from a couple of days to a week or more in some locations.
The main downside with this treatment is that many cats develop
hypothyroidism (an underactive thyroid) after treatment, which may damage
the kidneys. Fortunately in most cases the hypothyroidism is temporary,
but some vets who are prepared to give
I-131 treatment to cats who already have CKD believe that cats with CKD
should be treated with
L-thyroxine (a treatment for hypothyroidism) during this period
of stabilisation, so as to prevent further damage to the kidneys.
Feline hyperthyroidism: avoiding further renal
injury Broome MR discusses this.
Radioactive Iodine Treatment (I-131) Dosage
Cats used to be given a "standard" dose of 4-5 millicuries of radioactive
iodine which was too much for some cats and thus made them hypothyroid.
It is now thought the best approach is to tailor the dose to the individual cat.
Efficacy of low-dose (2 millicurie) versus
standard-dose (4 millicurie) radioiodine treatment for cats with
mild-to-moderate hyperthyroidism (2017) Lucy JM,
Peterson ME, Randolph JF,
Scrivani PV, Rishniw M,
Davignon DL, Thompson
MS & Scarlett JM
Journal of Veterinary Internal Medicine31(2) pp326-34
compared a lower dose to the standard dose. It found that "There
was no significant difference in prevalence of cats with persistent
hyperthyroidism between standard- and low-dose treatment groups at 3 and 6
months. Overt or subclinical hypothyroidism was more common in cats at 6
months after standard-dose 131I. No difference in incidence of
azotemia existed between groups, but cats treated with standard-dose
131I had higher creatinine concentrations and higher percent rises
in creatinine." The study concludes that the lower dose is safe and
effective, with a cure rate of over 95%.
Survival times for cats with hyperthyroidism
treated with a 3.35 mCi iodine-131 dose: a retrospective study
(2018) Vagney M, Desquilbet L, Reyes-Gomez E, Delisle F,
Devauchelle P, Rodriguez-Piñeiro MI, Rosenberg D & de
Fornel-Thibaud P Journal of Feline Medicine and Surgery
20(6) pp528-534 used a higher dose (3.35 millicurie), but
still lower than the standard dose of 4-5 millicuries was
effective for 97.9% of the cats in the study.
Radioactive Iodine Treatment (I-131)
Requirements
The cat needs to stop any medication for hyperthyroidism two weeks before
the I-131 treatment.
Because the treatment is radioactive, the cat needs to remain hospitalised
until s/he is at a safe level. The time period is determined by the
authorities where the treatment is performed, not by the treating
facility, and varies from country to country and state to state. It can be
as long as a week in some countries such as Australia, but is just a
couple of days in some other locations. During this period you cannot touch your
cat, so you could consider leaving an item of clothing with your smell on
to comfort your cat (take an old item because it cannot be returned).
Following the treatment, the cat must be kept indoors for two weeks and
away from pregnant women and children.
It is essential
to monitor cats after treatment for hyperthyroidism because so many of
them (15-49%, according to
Effects of feline hyperthyroidism on kidney function: a review
(2016) Vaske HH, Schermerhorn T & Grauer GF Journal of Feline Medicine
& Surgery18(2) pp55-9), will have worsening of their kidney
function.
However,
Effects of feline hyperthyroidism on kidney function: a review
(2016) Vaske HH, Schermerhorn T & Grauer GF Journal of Feline Medicine
& Surgery18(2) pp55-9) states that creatinine may
"continue to increase for 6 months after the cat becomes euthyroid,
while GFR can decrease for up to one month following attainment of
euthyroidism, and then will stabilize."
Your cat should
be monitored for at least six months once s/he is euthyroid. Since T4 is
often lowered in cats with non-thyroidal illness syndrome, TSH should also
be assessed.
What's the expected time for signs of feline
hyperthyroidism to resolve after treatment?
(2014) Peterson ME Insights Into Veterinary Endocrinology states "To monitor for iatrogenic hypothyroidism, we routinely run a serum
thyroid panel (i.e., total T4, T3, free T4, and TSH) at 1 and 3 months
after treatment. Most facilities recommend monitoring just the total T4
concentration, but this is not adequate for monitoring since many
hypothyroid cats will maintain a low-normal total T4 value, despite being
hypothyroid. Based on our studies, it's becoming increasing clear that
feline hypothyroidism can only be diagnosed by finding low to low-normal
T4 and T3 values in conjunction with high TSH values."
It is
particularly important
to monitor for hypothyroidism in cats treated for
hyperthyroidism because hypothyroidism can damage the kidneys.
Association of iatrogenic hypothyroidism with
azotemia and reduced survival time in cats treated for hyperthyroidism
(2010) Williams TL, Elliott J & Syme HM Journal of Veterinary
Internal Medicine24(5) pp1086-92 found that "iatrogenic
hypothyroidism appears to contribute to the development of azotemia after
treatment of hyperthyroidism, and reduced survival time in azotemic cats." Iatrogenic
means the hypothyroidism was caused by a
curative treatment for hyperthyroidism, i.e. surgery or I-131 treatment.
Fortunately in most cases hypothyroidism is not permanent.
2016 AAFP guidelines for the management of feline
hyperthyroidism (2016) Carney
HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A &
Rucinsky AR
Journal of Feline Medicine & Surgery18(5) pp400-416 state "Because 131I
predominantly damages hyperactive cells, permanent post-treatment
hypothyroidism is an uncommon sequela."
However, some
cats may need to be treated for hypothyroidism while it is present,
particularly CKD cats, in order to protect their kidneys. In some
cases the cat may need to be on treatment for hypothyroidism on a
permanent basis (in the form of a daily pill of levothyroxine).
When to start thyroid hormone replacement in cats
treated with radioiodine (I-131)
(2015) Peterson ME Insights Into Veterinary Endocrinology explains
when he treats for hypothyroidism.
Prognosis
Long
term health and predictors of survival for hyperthyroid cats treated with
iodine 131(2001)
Slater MR, Geller S, Rogers K Journal of Veterinary Internal Medicine15 pp47-51 is a study of the survival rate of
231 cats treated with the I-131 treatment which found that having
kidney disease adversely affected the chances of survival following this
type of treatment.
This is in part
because of the risks of I-131 treatment causing the opposite problem
of hypothyroidism.
Association of iatrogenic hypothyroidism with
azotemia and reduced survival time in cats treated for hyperthyroidism
(2010) Williams TL, Elliott J & Syme HM Journal of Veterinary
Internal Medicine24(5) pp1086-92 found that "Iatrogenic
hypothyroidism appears to contribute to the development of azotemia after
treatment of hyperthyroidism, and reduced survival time in azotemic cats."
(Iatrogenic hypothyroidism means hypothyroidism caused by I-131 or
surgery).
However,
hyperthyroidism
treatments are safe and effective in most cases.
Survival times for cats with hyperthyroidism treated with iodine 131,
methomazole, or both; 167 cases (1996-2003) (2006) Milner RJ1,
Channell CD, Levy JK & Schaer M Journal of the American Veterinary
Medical Association228(4) pp559-63 assessed various treatment
options. 14% of the cats in the study had CKD before treatment. The study
states "Age was
positively correlated (r = 0.4) with survival time, with older cats more
likely to live longer. Cats with preexisting renal disease had
significantly shorter survival times than did cats without preexisting
renal disease. When cats with preexisting renal disease were excluded,
median survival time for cats treated with methimazole alone (2.0 years;
interquartile range [IQR], 1 to 3.9 years) was significantly shorter than
median survival time for cats treated with 131I alone (4.0 years; IQR, 3.0
to 4.8 years) or methimazole followed by 131I (5.3 years; IQR, 2.2 to 6.5
years)." As you can see, the study refers to survival periods of a
year or often longer following treatment.
Effects of feline hyperthyroidism on kidney function: a review
(2016) Vaske HH, Schermerhorn T & Grauer GF Journal of Feline Medicine
& Surgery18(2) pp55-9 states "The reported median survival
time for cats with hyperthyroidism ranges from 1.6–4.0 years, while cats
classified as having CKD prior to treatment of hyperthyroidism have
shorter survival times of 0.5–2.0 years."
Survival of hyperthyroid cats is not affected by post-treatment azotemia
(2006) Wakeling J, Rob C, Elliott J, Syme H Journal of Veterinary Internal Medicine20
p1523 states that the "mild azotemia that can occur after treatment of
hyperthyroidism does not affect outcome... It would appear that there is
no indication to discontinue therapy for hyperthyroidism if mild azotemia
develops with therapy. The survival time after therapy is relatively long
in most cats considering that hyperthyroid cats represent geriatric pets."
Feline hyperthyroidism: avoiding further renal
injury Broome MR states "Most cats with concurrent CKD and hyperthyroidism will benefit from
the administration of subcutaneous fluids following the initiation of
antithyroid therapy."
If I had a
cat with hyperthyroidism, I would treat him or her.
Holistic Treatments
I know some people like to use holistic treatments so I am mentioning a
few here. I do not know anybody who has used them, and it must be accepted
that although some may possibly seem to work for a short while, since
hyperthyroidism is progressive, eventually you will need to use an
allopathic treatment; and while you've been delaying doing so, your cat's
kidneys have been deteriorating and you may have put your cat at risk of
heart problems and strokes.
If you wish to take the homeopathic route, you must use a qualified vet,
not just buy the remedy over the counter and try to treat yourself.
Homeopathic treatment of feline hyperthyroidism
(2006) Dobias P Presentation to the 31st World Small Animal
Veterinary Association World Congress reports on the use of a homeopathic
remedy, Natrum Muriaticum, to treat hyperthyroidism in 13 cats. Results
were mixed, but it did appear to be effective for eight (61.5%) of the
cats. It appeared to be most effective when used in relatively young cats
(average age of the group who responded positively to the treatment was
7.4) who were diagnosed early.
L-carnitine may be of some limited use in controlling the symptoms of
hyperthyroidism, at least initially, but like ther holistic treatments it
will not reduce your cat's T4 levels or the size of the tumour causing the
hyperthyroidism.
Alternative medical treatments for hyperthyroid cats
(2012)
Peterson ME Insights Into Veterinary Endocrinology
says "Personally, I have never seen a cat treated with
one of these natural alternative medications that was cured or has
clinical signs resolve, at least on a long-term basis. Most of the active
ingredients in the herbs act by supposedly lowering TSH secretion, but
circulating TSH is already low in these cats with hyperthyroidism. It is also important to
remember that cats with hyperthyroidism usually have one or more thyroid
adenomas and, less commonly, thyroid carcinoma. No one has ever
documented shrinkage of the thyroid tumor or significant lowering of the
serum T4 value after these alternative medications."
Diet
Diet and nutritional management for hyperthyroid cats
(2011) Peterson ME
Insights into
Veterinary Endocrinology states that hyperthyroid cats usually benefit from a
diet composed of around 50% protein. He writes "This absolute requirement
for dietary protein intake in cats is critically important when
formulating a diet for hyperthyroid cats, in which protein catabolism and
muscle wasting is universally present."
The best diet to feed hyperthyroid cats
(2011) Peterson ME
Insights into
Veterinary Endocrinology discusses weight and muscle loss in
cats and
says, "Like normal and diabetic cats, I believe that it makes sense to
feed most hyperthyroid cats a diet composition close to what they would be
getting in the wild. That would be a diet composed of approximately 50-60%
protein, 5-10% carbohydrates, and 30-50% fat."
Hyperthyroidism
may cause a number of different problems which may need additional
treatment, as follows:
Hypertension
Many cats with
hyperthyroidism also have
hypertension, which may improve once the hyperthyroidism is under
control. If your cat continues to have hypertension despite getting the hyperthyroidism under control, you
should discuss the need for medication with your vet. Untreated
hypertension is very dangerous.
Heart Problems
If your cat has heart problems that do not resolve following treatment for
hyperthyroidism, your cat may need treatment for the heart.
Pet Place states "Atenolol can be used in
the management of an overactive thyroid gland (hyperthyroidism) in cats,
in which excess thyroid hormone adversely affects the heart."
Please read the
Heart Problems
page (which includes information on
atenolol) and discuss how best to manage them with your vet.
Urinary Tract Infections
Hyperthyroidism: a view from the urinary tract
(2010) Grauer GF CVC in Kansas City Proceedings says "urinary
tract infection (UTI) can be consequences of either hyperthyroidism or
CKD..."Urine cultures should be obtained as part of the workup of
both hyperthyroidism and CKD. In either case, a concurrent UTI should be
managed as a complicated UTI with long-term antibiotic treatment based
on culture and sensitivity results."
Hyperthyroidism is not a risk factor for subclinical
bacteriuria in cats: a prospective cohort study (2020) Peterson
ME, Li A, Soboroff P, Bilbrough GE & Rishniw M Journal of Veterinary
Internal Medicine Epub ahead of print found that hyperthyroid cats do
not appear to be at higher risk of subclinical (i.e. the cat does not show
any signs) UTIs and states that it is not necessary to check urine in cats
without signs of a UTI.
TREATING YOUR CAT WITHOUT VETERINARY ADVICE CAN BE
EXTREMELY DANGEROUS.
I have
tried very hard to ensure that the information provided in this website is
accurate, but I am NOT a vet, just an ordinary person who has lived
through CKD with three cats. This website is for educational purposes
only, and is not intended to be used to diagnose or treat any cat. Before
trying any of the treatments described herein, you MUST consult a
qualified veterinarian and obtain professional advice on the correct
regimen for your cat and his or her particular requirements; and you
should only use any treatments described here with the full knowledge and
approval of your vet. No responsibility can be accepted.
If your cat
appears to be in pain or distress, do not waste time on the internet,
contact your vet immediately.
This site was
created using Microsoft software, and therefore it is best viewed in
Internet Explorer. I know it doesn't always display too well in other
browsers, but I'm not an IT expert so I'm afraid I don't know how to
change that. I would love it to display perfectly everywhere, but my focus
is on making the information available. When I get time, I'll try to
improve how it displays in other browsers.
This site is a labour of love. Please do not steal from me by taking credit for my work.
If you wish to
link to this site, please feel free to do so. Please make it clear that
this is a link and not your own work. I would appreciate being informed of
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