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HYPERTHYROIDISM (OVERACTIVE THYROID)

 

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Home > Related Diseases > Hyperthyroidism

 


Overview


  • Hyperthyroidism is relatively common in older cats.

  • It is caused by a tumour, which in most cases is benign (non-cancerous).

  • It can cause a lot 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.

  • 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 Hyperthyroidism?                                                                                   Back to Page Index


What Happens in Hyperthyroidism


The thyroid gland is situated at the front of the neck, and is responsible for the regulation of metabolism. Imbalances may sometimes occur, so the thyroid becomes underactive or overactive.

 

Hyperthyroidism means that the thyroid is overactive Like CKD, hyperthyroidism (sometimes abbreviated as hyperT) is very common in older cats. It is usually caused by a tumour known as an adenoma which in most cases is benign (i.e. the tumour is not cancerous). In most cases, both lobes of the thyroid gland are affected.

 

Hyperthyroidism speeds up metabolism, and as a result all the body's processes speed up. Untreated hyperthyroidism can cause hypertension (high blood pressure), heart problems (according to Gulf Coast Veterinary Specialists, 87% of hyperthyroid cats have some degree of hypertrophic cardiomyopathy), and can also affect phosphorus and calcium levels and cause hyperparathyroidism. Fortunately, if a cat has heart problems caused by hyperthyroidism, treating the hyperthyroidism can often reverse the heart problems. 

 

Hyperthyroidism and CKD


One of the body's metabolic processes which is speeded up by hyperthyroidism is bloodflow through the kidneys, which affects the glomerular filtration rate (GFR), a measure of kidney function. In addition, the loss of muscle mass which often accompanies hyperthyroidism can reduce creatinine levels (since creatinine is a by-product of muscle). This can make it 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. Once the hyperthyroidism is treated, the true function of the kidneys becomes apparent, and it is often at this point that CKD is finally diagnosed.

 

This might tempt you not to try to get the hyperthyroidism under control, or to only get it partially under control, in an attempt to spare the kidneys. However, this is not a good idea, because of the other effects of hyperthyroidism on the body (see above).

 

Complications and consequences of feline hyperthyroidism (2003) is a presentation by CT Mooney to the 2003 World Small Animal Veterinary Association World Congress which explains more about the effects of hyperthyroidism on the body.

 

Hypothyroidism and Euthyroid Sick Syndrome


Occasionally a cat may appear to have the opposite problem to hyperthyroidism, i.e. hypothyroidism. This means the thyroid is underctive, so metabolic processes are slower than they should be.

 

True hypothyroidism is common in dogs but is extremely rare in cats. In the case of a sick cat, such as a CKD cat, if bloodtests indicate that a cat appears to be hypothyroid, it is more likely that the cat has a condition known as "euthyroid sick syndrome" or ESS. This occurs when T4 levels appear lower than they actually are because of another concomitant disease. This does not usually need any treatment, because if you get the accompanying disease under control, the thyroid hormones should return to a normal level.

 

ESS may also be caused by starvation, so if your cat has not been eating, this might be a factor.

 

Hypothyroidism may also appear after treatment for hyperthyroidism. Usually this is mild and transient and will not need treatment, see below.

 

Newman Veterinary discusses euthyroid sick syndrome.

Emedicine also discusses euthyroid sick syndrome in humans but the principles are the same.

 


Possible Causes                                                                                                   Back to Page Index


 

It may surprise you to learn that hyperthyroidism was unknown in cats until the late 1970s. Therefore there is much speculation about the possible causes. This section contains information on current research into possible (but as yet unproven) causes.

A Food Connection?


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 Association 224 (6) pp 879-886 was a study during which the records of just under 170,000 cats over a 20 year period were examined. The study indicates that the consumption of canned food appears to carry an increased risk (almost four times the risk compared to cats who ate dry food) of developing hyperthyroidism, and that the risk increases for every year that the cat eats canned food. 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), a potential endocrine disrupter. Males appeared to be at risk only if they ate 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 from pop-top cans or other types of cans. 

 

Whilst this is a worrying study, generally speaking CKD cats do better on a wet food diet (see Nutritional Requirements). A possible solution might therefore be to feed food in foil pouches or plastic 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 Food Standards Agency has more information on this.

 

Risk factors for feline hyperthyroidism in the UK (2009) Everard A, Brodbelt D, Elliott J, Syme H Journal of Small Animal Practice 50 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.

 

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 Association 217(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 below).

 

Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats (2004) White HL, Freeman LM, Mahony O, Grahan PA, Hao Q & Court MH American Journal of Veterinary Research 65(5) pp586-91 found that soy in a healthy cat's diet may increase T4 levels.

 

A Flame Retardant Connection?


Elevated PBDE levels in pet cats: sentinels for humans? (2007) Dye JA, Venier M, Zhu L, Ward CR, Hites RA & Birnbaum LS Environmental Scientific Technology 41(18), pp6350-56, reports that there may be a correlation between the rise in feline hyperthyroidism and the increased use of chemicals known as polybrominated diphenyl ethers (PBDEs) in the home. PBDEs are commonly used as flame retardants. They are thought to mimic thyroid hormones, and thus may be a factor in the development of hyperthyroidism. In this study, 23 cats (eleven with hyperthyroidism) had levels of PBDE in their bodies measured. 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 above food study, indoor cats who favoured fish-flavoured tinned 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? (2007) Mensching DA, Ferguson D, Bordson G, Scott J, Piwoni M, Beasley V is a summary of the goals of a two year study (completed in June 2007) which was also investigating the significance of PBDEs in cat food and the possible link between PBDEs and the development of hyperthyroidism in cats. Scroll down to pages S-102-3.

 


Symptoms                                                                                                               Back to Page Index


 

The classic symptom of hyperthyroidism in cats is that the cat loses weight despite eating a lot. Some of the other symptoms of hyperthyroidism which your cat may exhibit are similar to those seen in CKD, including:

  • increased urination

  • increased thirst

  • a greasy coat

  • spiky fur

  • vomiting

  • hoarseness

  • restlessness

  • 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).

  • hypertension (high blood pressure) - Hypertension in cats with chronic renal failure or hyperthyroidism (1990) Kobayashi DL, Person 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.

However, just to confuse matters, some hyperthyroid cats have "apathetic hyperthyroidism", and become lethargic and lose their appetite instead. Gulf Coast Veterinary Specialists say that up to 30% of hyperthyroid cats are anorexic.

 


Diagnosis                                                                                                                Back to Page Index


 

Physical Examination


One vet told me he has found that if he pinches a cat's ear, its pulse will slow down, except in hyperthyroid cats. I don't have a hyperthyroid cat at the moment so I haven't been able to test this myself. Obviously, this is not a proper method of diagnosis, so if you suspect hyperthyroidism, you should take your cat to the vet for tests.

 

The vet can often feel a nodule in the thyroid. The vet may also detect high blood pressure and/or a faster heart rate than usual, while the heart may be enlarged. However, these tests also cannot definitely diagnose hyperthyroidism, so blood tests should also be run.

 

Blood Tests


Routine blood tests are usually run, which may show elevated liver values such as ALT. Cats with hyperthyroidism may have HCT or PCV levels in the higher end of the range. They may have low potassium levels.  Calcium and phosphate homeostasis in hyperthyroid cats: associations with development of azotaemia and survival time (2012) Williams TL, Elliott J & Syme HM Journal of Small Animal Practice 53(10) pp561-71 found that approximately 60% of the hyperthyroid cats in the study had elevated parathyroid hormone levels, which decreased following treatment in cats who did not have CKD.

 

Additional blood tests to check thyroid function will also be run, which often need to be sent to an external laboratory for analysis. These tests are as follows:

 

T4 Levels


In terms of specific tests of thyroid function, the most usual test is for a hormone called thyroxine, which is commonly known as T4 or Total T4. Below I provide a rough guide to T4 levels but the laboratory used by your vet will provide its usual reference range.

 

T4 Level: US Values T4 Level: International Values Interpretation
Below 1.0 ug/dl Below 13 nmol/L Low (Below normal)
1.0 - 5.0. ug/dl 13 - 64 nmol/L Normal
2.5 - 5.0 ug/dl 32 - 64 nmol/L Grey zone
Over 5.0 ug/dl Over 64 nmol/L Consistent with hyperthyroidism

 

Cats with a low T4 level usually have Euthyroid Sick Syndrome or, if they are already on treatment for hyperthyroidism, they may have been overtreated (i.e. the overactive thyroid has been treated too aggressively so the opposite problem of an underactive thyroid occurs)..

 

As you can see, there is a grey zone area. This is because the normal ranges do not apply to older cats (over the age of 10), and about 10% of cats who fall into this range will in fact have hyperthyroidism despite having a theoretically normal T4 level. Be sure your vet allows for this when interpreting test results.

 

T3


Unfortunately, as the Winn Feline Foundation mentions, a small number of cats with hyperthyroidism may appear to have normal T4 levels because levels may fluctuate throughout the day (Gulf Coast Veterinary Specialists say that T4 levels can actually fluctuate up to 24% over 24 hours), so you may just happen to test when the level looks normal.  Cats with other conditions, including CKD, may also have Euthyroid Sick Syndrome, which may also reduce T4 levels. Therefore many vets also routinely check levels of another hormone, tri-iodothyronine (T3). In a hyperthyroid cat, levels of both T3 and T4 are usually elevated, although sometimes only one of them is higher than normal.

 

Free T4 by Equilibrium Dialysis


Occasionally both T4 and T3 will appear normal, especially in cats with mild CKD, but you still suspect your cat may have hyperthyroidism. Diagnosis of hyperthyroidism in cats with mild chronic kidney disease (2008) Wakeling J, Moore K, Elliott J & Syme H Journal of Small Animal Practice 49(6) pp287-94 found that the cats in the study with both CKD and signs consistent with hyperthyroidism nevertheless had normal T4 levels. This happened to us with Ollie: 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 should not be relied upon in isolation (it should be run in conjunction with T4 and T3 tests), but it can assist with making a diagnosis. In 2008 Antech Diagnostics stated: "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." In the study mentioned in the previous paragraph, free T4 was elevated in 15 of the 16 cats with chronic kidney disease who had signs of hyperthyroidism but normal T4 levels, and in 14 of these cats it was over 40 mmol/L.

 

Ollie's Free T4 test came back indicating that he did in fact have hyperthyroidism. Once he was placed on medication, he improved greatly.

 

Michigan State University has a helpful overview of the significance of the various tests.

Newman Veterinary also has some information on the use of the free T4 by equilibrium test (click on hyperthyroid diagnosis, an update).

Idexx Laboratories launched a new free T4 test in 2012 which is not performed by equilibrium dialysis. This means it can be turned around more quickly than the usual test and apparently it is just as reliable, but I do not know anybody who has used it yet.

Technetium Scan


Some centres offer a technetium scan, which uses a radioactive solution which 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, which is particularly important if you are considering surgery or I-131 treatment.

 

Gulf Coast Veterinary Specialists has some information about this test.

The International Cat Care also has some information.

 


Treatments                                                                                                                                        Back to Page Index


 

It is essential to treat hyperthyroidism because it is a strain on the cat's body, particularly the heart. However, you do need to proceed cautiously. If a cat has both CKD and hyperthyroidism, the hyperthyroidism can actually mask the effects of CKD; so once the hyperthyroidism is treated the CKD can then appear to worsen (though what is actually happening is that the true degree of renal function is becoming apparent). In some cases, previously undiagnosed CKD may actually appear for the first time once hyperthyroidism is treated.

 

Generally speaking, the goal is a T4 in the low 2s (USA) or around 25-30 (international) but some vets prefer to keep it a little higher than this in a CKD cat.

 

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.

 

Diet


In Diet and nutritional management for hyperthyroid cats (2011) Dr ME Peterson 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."

 

For cats with both CKD and hyperthyroidism, Dr Peterson states in Dietary management of cats with kidney disease after treatment with radioiodine for hyperthyroidism (2013) that reducing protein should be avoided for cats in IRIS stages 1 and 2, but that phosphorus control is very important. This is in line with the recommendation for cats with CKD only.

 

Hill's y/d


In October 2011 Hill's introduced a new food called Hill's Prescription Diet y/d. It is available in both canned and dry versions. The food contains reduced levels of iodine, and Hill's claims that feeding the food exclusively for three weeks can control hyperthyroidism.

 

Hill's state that this food should only be fed to the cat with hyperthyroidism, who can eat this food only and nothing else. Healthy cats with normal thyroid function should not be allowed to eat the food because it could cause problems.

 

The canned food is low in phosphorus (0.59%) and protein (34.1%). The dry food is a little higher (phosphorus 0.66%, protein 36.3%) but still low.

 

I'm not entirely convinced that a food can be as effective as the other treatment options discussed below, but I'll wait to hear of people's experiences with it.

 

DVM News Magazine reports on the new Hill's foods.

Dr ME Peterson gives his opinion of Hill's y/d.

 

Drugs: Methimazole and Carbimazole


 

Medications are used to regulate the production of thyroid hormone. The two main treatments are methimazole or carbimazole. In the USA you will normally be offered methimazole, whereas in the UK you are more likely to be offered carbimazole.

 

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.  

 

Antithyroid drug treatment for hyperthyroidism: brand name, generic or compounded drug? (2012) Peterson ME gives an overview of the various drugs used for hyperthyroidism in cats and the forms in which they are available.

 

The Importance of Starting With a Low Dose


When you first start thyroid medication, it is very important to use a low dose and increase only as necessary, because this reduces the risks of an adverse reaction. Since treating hyperthyroidism may unmask hidden CKD, this also gives you the chance to watch for signs of CKD developing, or worsening in an already diagnosed CKD cat, which can help you avoid a crash.

 

Your goal is to get the hyperthyroidism firmly under control, aiming for the T4 to be within the lower end of the normal range (see table above), but you do not want to over medicate, which would cause the opposite problem of hypothyroidism (an underactive thyroid). 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 Medicine 24(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." That is to say, treating hyperthyroidism to the extent that the thyroid became underactive with increased BUN and creatinine led to a reduced survival time.

 

Methimazole


Methimazole is the most commonly used thyroid medication in the USA, and was recently introduced in the UK. The trade name for the feline version of methimazole is Felimazole in both the USA and the UK but in the USA you might possibly still be offered the human version, known as Tapazole.

 

Methimazole is a relatively simple way of treating hyperthyroidism but it can have side effects such as lack of appetite, vomiting and even anaemia in some cats; plus it only controls the condition, it will not cure it. Veterinary Partner discusses possible side effects, including itching, particularly on the face. Cats on methimazole may also have elevated liver enzymes and low white blood cells. Advanced Veterinary Medical Imaging has photos of hyperthyroid cats with facial lesions caused by scratching their itchy faces, plus a photo of a cat with jaundice due to liver involvement.

 

Efficacy and safety of transdermal methimazole in the treatment of cats with hyperthyroidism (2004) Sartor LL, Trepanier LA, Kroll MM, Rodan I, Challoner L J Journal of Veterinary Internal Medicine 18(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. Drs Foster & Smith have a video about giving the medication in this way. If you do use transdermal medications, Hyperthyroid cats and transdermal methimazole is a helpful video which shows how one person administers transdermal medications to his cat.

 

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 liver enzyme called CYP2D6 which helps to clear mirtazapine from the body.

The University of Maryland Medical Center (search for both drugs) has some information about this.

 

Methimazole Dosing


Try to start at the lowest dose possible and increase only as necessary, because this reduces the risks of an adverse reaction. Many vets suggest relatively high doses, but it is usually best to start at a dose of 2.5mg once a day for the first two weeks. Try to make the doses evenly spaced. For cats with pre-existing CKD, an even lower starting dose is suggested in 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, which 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".

 

Diagnostic and treatment options for feline hyperthyroidism (2003), a presentation by Richard Nelson to the World 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).

 

Cats with chronic renal failure (CRF) - how different than CRF in dogs? (2007) a presentation by Drs DJ Chew & SP DiBartola to the World Small Animal Veterinary Association World Congress, also recommends starting at a dose of 2.5mg once a day for the first two weeks, at which point repeat bloodwork should be run to check kidney and thyroid function. If kidney function is stable, the methimazole dose can be very gradually increased every two weeks if necessary to as much as 5mg three times a day, until T4 levels are normal.

 

Drs Chew and DiBartola recommend stopping methimazole if the cat's kidney function worsens while giving methimazole. However, Veterinary Practice News refers to another study, Survival of hyperthyroid cats is not affected by post-treatment azotemia (2006) Wakeling J, Rob C, Elliott J, Syme H Journal of Veterinary Internal Medicine 20 p1523, which 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."

 

How to dose and monitor hyperthyroid cats on methimazole (2012) Dr MS Peterson explains more about dosing and monitoring cats on methimazole.

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 your cat does badly on methimazole, 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.

Plumb's Veterinary Drug Handbook also mentions this.

Feline hyperthyroidism: spectrum of clinical presentations and response to carbimazole therapy (2000) Bucknell DG Australian Veterinary Journal 78(7) mentions that there is less chance of side effects with carbimazole than with methimazole.

 

Carbimazole - UK


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.

 

Therefore you will most probably be offered the newish veterinary version of carbimazole called Vidalta (although it is also possible that your vet will offer you methimazole with the brand name of Felimazole). Personally I would always opt to use carbimazole over methimazole.

 

Vidalta currently is only available in 10mg or 15mg tablets which are slow release, so they should not be crushed. Some people were concerned at 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), Dr MS Peterson 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 above for dosing.

 

Carbimazole - USA


Carbimazole can be obtained in the USA from compounding pharmacies (see Tips on Medicating Your Cat).

 

Carbimazole Dosing


Similarly to methimazole, a cautious starting dose is recommended 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). Carbimazole may be given up to three times a day if necessary.

 

Having said that, one study, Pharmacokinetics of controlled-release carbimazole tablets support once daily dosing in cats (2008) Frénais R, Burgaud S, Horspool LJ Journal of Veterinary Pharmacology and Therapeutics 31(3) pp213-9, found that Vidalta was effective when given only once a day. A further study, Clinical efficacy and safety of a once-daily formulation of carbimazole in cats with hyperthyroidism (2009) Frénais R, Rosenberg D, Burgaud S, Horspool LJ Journal of Small Animal Practice 50(10) pp510-5 also concluded that once daily dosing was effective. These studies were conducted by the manufacturer of Vidalta.

 

For cats with pre-existing CKD, a lower starting dose is suggested in 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, which 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".

Surgery


The second treatment method for hyperthyroidism is surgical removal of the thyroid gland, which is known as thyroidectomy. Although this is usually successful, it is of course invasive and too much of the gland may be removed, resulting in the opposite problem, an underactive thyroid (hypothyroidism). This treatment may be risky for cats with undiagnosed CKD (see below).

 

Radioactive Iodine Treatment (I131)


The third possible treatment for hyperthyroidism is radioactive iodine treatment (I-131). This destroys the overactive thyroid tissue and has no major side effects, but it is only available in a limited number of locations, and the cat has to be hospitalised for a period ranging from a few days to as long as six weeks in a few cases. This treatment may be risky for cats with undiagnosed CKD (see below). 

 

Homeopathic Treatment


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.

 

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.

Related Health Concerns


Most of the symptoms of hyperthyroidism should improve as it is brought under control with treatment.

 

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.

 

If your cat also has heart problems that do not resolve, please read the Heart Problems page and discuss how best to manage them with your vet.

 

A recent study, The prevalence of hypocabalaminaemia in cats with spontaneous hyperthyroidism (2011) Cook AK, Suchodolski JS, Steiner JM & Robertson JE Journal of Small Animal Practice 62(2) pp101-6, found that hyperthyroid cats may have low cobalamin (Vitamin B12) levels. I would therefore discuss Vitamin B12 supplementation with your vet.


Treatment Cautions                                                                                              Back to Page Index


 

Treating hyperthyroidism, whilst essential, may carry some risks. This is because, if a cat has both CKD and hyperthyroidism, the hyperthyroidism can mask the effects of CKD; but once the hyperthyroidism is treated the CKD can then appear to worsen (in fact, the true level of kidney function is being revealed). Even in cats who are not known to have CKD, there is a potential risk, because previously undiagnosed CKD may appear for the first time. This is not a small risk: according to Hyperthyroidism and the kidney (2006) Langston CE & Reine NJ Clinical Techniques in Small Animal Practice 21(1) pp17-21, "overt renal failure occurs in approximately 30% of cats treated for hyperthyroidism".

 

It is hard 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 & Surgery 10(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, validating the necessity of a methimazole trial prior to definitive therapy".

 

Because of this, it is safest to start treating hyperthyroidism with medication, because, should you see any signs of problems, you can adjust the dose in an attempt to balance the conflicting needs of the kidneys and the thyroid, and avoid a crash, whereas this is not possible with the surgical and radioactive iodine treatments, where the treatment is complete and no finetuning is possible. Feline hyperthyroidism 2009 (Proceedings) (2009) Graves TK explains more about using medication as the first choice and recommends doing this for at least thirty days.

Surgery or I-131 treatment are usually considered unsuitable for cats who already have CKD. They may also be risky for cats with undiagnosed CKD, especially if your cat is overtreated and develops hypothyroidism. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism (2010) WilliamsTL, Elliott J & Syme HM Journal of Veterinary Internal Medicine 24(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." That is to say, treating the hyperthyroidism to the extent that the thyroid moved into the underactive category (hypothyroidism) and BUN and creatinine increased led to a reduced survival time.

 

Of course, not every hyperthyroid cat treated via these methods will become hypothyroid; and in many cases, if a cat does become hypothyroid following treatment, it will not be permanent: Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine (2009) van Hoeka I, Lefebvreb HP, Peremansa K, Meyera E, Croubelsa S, Vandermeulena E, Kooistrac H, Saunders JH, Binsta D & Damineta S Domestic Animal Endocrinology 36(1) pp45-56 found that "significant changes in kidney function occur within 4 weeks post-treatment and none thereafter", so your cat's condition should at least stabilise rather than continue to worsen. However, 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 MT discusses this.

If you do decide to opt for surgery or I-131 treatment, I would always recommend a trial using thyroid medications first to see what impact treating the hyperthyroidism has on your cat's kidney function. If you are in the USA, you may also wish to consider using the iohexol clearance test, which measures your cat's GFR (see Methods of Early Detection), which may help you decide if your cat is at risk of CKD following treatment, though this is not reliable at detecting all cats at risk of developing CKD. Even if you take these additional precautions and all looks well, there is still a chance of unmasking CKD if you move on to I-131 treatment or surgery after a medication trial.

 

Washington State University College of Veterinary Medicine recommends treating with medication in the first instance.

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 Association 36 pp215-233 is a study on the use of methimazole in cats and its effects on unmasking CKD.

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 Medicine 15 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.   

 


Links                                                                                                                                                      Back to Page Index


 

Mar Vista Vet has helpful information on symptoms and treatments.

 

Long Beach Animal Hospital has an overview of hyperthyroidism in cats.

 

Winn Feline Foundation has an overview of hyperthyroidism.

 

Pet Education has a helpful article about hyperthyroidism.

 

Drs Foster and Smith give examples of three cats who were each given a different treatment plan.

 

Pet Place has some information about hyperthyroidism.

 

Current concepts on diagnosing and managing thyroid disease in dogs and cats roundtable discussion (2010)  NAVC Clinician's Brief gives an overview of hyperthyroidism.

 

Feline hyperthyroidism is a paper by Dr RM Hardy from the University of Minnesota College of Veterinary Medicine.

 

Advanced Veterinary Medical Imaging - more information on the treatments available for feline hyperthyroidism.

 

Feline HyperT Group is a support group for people with hyperthyroid cats, where you can obtain feedback on treatments, and support on living with hyperthyroidism.

 

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This page last updated: 22 September 2013

Links on this page last checked: 03 April 2012