Phosphorus Imbalances

Calcium Imbalances

Secondary Hyperparathyroidism, Including Calcitriol and Cinacalcet (Sensipar)




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Home > Treatments > Phosphorus, Calcium and Secondary Hyperparathyroidism



  • Imbalances of phosphorus and calcium levels within the body are very common in CKD cats.

  • Eventually this can lead to a serious condition known as secondary hyperparathyroidism.

  • This page discusses the control of phosphorus and calcium levels in order to reduce the risk of secondary hyperparathyroidism.

  • It also discusses treatments which may be used if secondary hyperparathyroidism is present.

  • Please see the Diagnosis section for more information on these issues.

Phosphorus Imbalances


High Phosphorus Levels

In most cases, CKD cats have high phosphorus levels. High phosphorus levels can make the cat feel bad and may make the CKD progress faster. For a CKD cat, being within the normal range is not sufficient: if your cat's phosphorus level is over  6 mg/dl (USA) or 1.9 mmol/L (international), it is too high and you need to get it under control.


This important topic has its own page here.


Low Phosphorus Levels

Low phosphorus levels (below 3 US or 1.0 international) are extremely uncommon in CKD cats, though are occasionally seen in diabetic cats on insulin.  If the phosphorus level is too low as a result of using phosphorus binders, you can reduce the dose of binder which you are giving.

Calcium Imbalances


High Calcium Levels

As the Diagnosis section explains, high calcium levels in CKD cats are not normally a problem unless ionised calcium levels are also high. However, there is an exception to this rule. If calcium multiplied by phosphorus is higher than 60-70 in US values or 5 in international values, it increases the risk of calcification, in which case you should ask your vet about trying to control total calcium levels.


If ionised calcium levels are high, you definitely need to take action.


Metabolic acidosis can contribute to hypercalcaemia, so if your cat has metabolic acidosis, treating it can help reduce calcium levels.


Although it is fairly rare, high calcium levels may be caused by cancer, so if the cause of your cat's hypercalcaemia is unknown, especially if you find the treatments below ineffective, it is worth asking your vet to test for cancer.


Antech Diagnostics (2000) has information on hypercalcaemia and how to treat it from Dr D Chew.


Simple Treatments

Advanced Treatments

Simple Treatments

These treatments are simple in that you will probably be using them for other reasons anyway, or they do not require much effort to implement.


Subcutaneous Fluids (Sub-Qs)

Keeping your cat properly hydrated is one of the main goals of CKD treatment, so most cats with creatinine over 3.5 (300 international) will be receiving sub-Qs for this reason. As it happens, giving sub-Qs may also reduce calcium levels, though it is not usually recommended to use sub-Qs solely in order to control calcium levels.


Normally cats on sub-Qs with high calcium levels are given saline solution rather than Lactated Ringers Solution (LRS). This is for two reasons: firstly, saline solution has no added calcium, and secondly, saline solution promotes calcium excretion in the urine. Alternatively, Normosol-R might be suitable. If you are using LRS, discuss switching with your vet.


Phosphorus Binders

Since phosphorus and calcium levels within the body are closely related, reducing elevated phosphorus levels may also help reduce calcium levels. If your cat's phosphorus level is over 6 mg/dl (USA) or 1.9 mmol/L (international), you need to take steps to control it, see All About Phosphorus. If you are using phosphorus binders, never use calcium-based phosphorus binders in a cat with elevated calcium levels.


Limiting Treatments Containing Calcium or Vitamin D

Slippery elm bark contains calcium, so it is probably safer not to use it if your cat has hypercalcaemia.


Some people have found that using lactulose to control constipation has led to hypercalcaemia in their cats. This may be coincidence, since hypercalcaemia is not uncommon in CKD cats, but you may wish to avoid lactulose if your cat already has hypercalcaemia, and consider alternative treatments if your cat has developed hypercalcaemia since beginning lactulose. See All About Constipation for more information and alternative treatments for constipation.


Also check for any supplements containing Vitamin D, e.g. Hi-Vites. Also ensure that you are not feeding a food high in Vitamin D.


Dietary Changes

It is worth checking which food you are feeding because some contain more calcium than others. The body needs a balance between phosphorus and calcium, so foods usually have a particular ratio of calcium to phosphorus. The minimum is 1:1, though for healthy cats a higher ratio of calcium to phosphorus is not normally a problem. Normal commercial diets tend to have a ratio of around 1.2:1 (calcium to phosphorus). Prescription kidney diets tend to contain more calcium in ratio terms because they are usually focused on keeping phosphorus levels low (Hill's k/d has a level of 1.7:1 canned and 1.43 dry. Purina NF is 1.68:1 for the dry and 1.36:1 for the canned. Royal Canin has much higher levels, at around 2:1) but the overall amount of calcium is still usually low.


If your cat is hypercalcaemic and in IRIS stages 1 and 2, discuss with your vet whether to feed a normal commercial diet with a lower calcium content rather than a prescription kidney diet. Cats in these stages do not normally need a low protein diet, see Nutritional Requirements, though it must not be forgotten that a prescription kidney diet has many features other than merely low protein, see Which Foods. Your vet can help you decide what to focus on. If your cat does require a prescription kidney diet, I would check the calcium:phosphorus ratio carefully and aim for a food with a lower calcium:phosphorus ratio, assuming your cat will eat it. You might wish to add a food to it which is relatively low in calcium, in order to adjust the calcium:phosphorus ratio. You should also avoid feeding acidified diets.


Nutritional-management-of-idiopathic hypercalcaemia in cats (2012) Peterson M discusses ways to treat idiopathic hypercalcaemia (idiopathic means no obvious cause can be found).



Increased fibre in the diet may help to reduce calcium levels by binding with the calcium. One food that may help with this is Hill's w/d, since it has added fibre. The canned version of this food would be suitable for most CKD cats, with a phosphorus level of 0.68% and a protein level of 39.60%. The phosphorus level in the dry food is a little high at 0.89%, with a protein level of 38.60%, but this might still be acceptable if your cat's phosphorus levels are under control.


You need to be careful when choosing a fibre. One type of fibre called fructooligosaccharides (FOS), often contained in probiotics, may actually cause elevated calcium levels. See Nutritional Requirements for more information on fibre.


Advanced Treatments

These treatments have pros and cons, and some of the cons are potentially serious. These treatments may nevertheless be necessary in a small number of cases. Discuss with your vet.



Corticosteroids such as prednisolone may sometimes be used to control high calcium levels. Using steroids can have certain undesirable side effects, see steroids. The Merck Veterinary Manual has more information about using steroids for this purpose.



Calcitriol is a hormone produced by the kidneys which helps to regulate parathyroid hormone (PTH). Some people use calcitriol as a supplement to try to control secondary hyperparathyroidism.


Although calcitriol may cause calcium levels to rise, in some cases it may be used to try to reduce calcium levels. However, this should only be attempted if ionised calcium is above midrange (or even at the top of the normal range), and an intermittent dosing schedule should be used. The causes and consequences of feline hypercalcemia (2009) Cook AK Presentation to the ACVIM Forum explains more about treatments for hypercalcaemia and mentions the use of calcitriol in CKD cats who have both elevated calcium and elevated ionised calcium levels. It states that if ionised calcium levels increase while using calcitriol for this purpose, it must be discontinued immediately.


See below for more information on calcitriol.



Lasix (furosemide) is a loop diuretic which reduces calcium levels because it increases the excretion of calcium via the kidneys. Diuretics are commonly used in heart disease but they are hard on the kidneys, and therefore not ideal for a CKD cat, so I would not use Lasix unless you absolutely had to.


Bisphosphonate Drugs

If (and only if) ionised calcium levels are at least 25% above the normal upper limit, you could ask your vet about using bisphosphonate drugs. One drug in this family is called alendronate (Fosamax). It is commonly used to treat humans with osteoporosis, and may be recommended for CKD cats for similar reasons i.e. so the bones absorb calcium, which then reduces calcium levels in the blood.


This is a last resort treatment - Plumb's Veterinary Drug Handbook states that "because of a lack of experience, the drug is not recommended for use in human patients with severe renal dysfunction." 


The usual starting dose is 10mg for cats. This is only given once a week but the cat must stay upright for at least 15 minutes after taking it so as to avoid oesophageal damage. Always follow the treatment with a water chaser. Plumb's suggests buttering the cat's lips to encourage swallowing. If you are using ranitidine (Zantac 75) intravenously for excess stomach acid (highly unlikely outside a hospital setting), be aware that it doubled the effect of alendronate in one human study. Ionised calcium levels should be checked regularly, starting four weeks after the first dose, and the dose should be adjusted if necessary .


Another bisphosphonate is pamidronate. Uses and effectiveness of pamidronate disodium for treatment of dogs and cats with hypercalcemia (2005) Hostutler RA, Chew DJ, Jaeger JQ, Klein S, Henderson D, DiBartola SP Journal of Veterinary Internal Medicine 19(1) pp29-33 found it appeared to be safe and effective. However, pamidronate has to be given intravenously and may cause electrolyte imbalances, plus it may be contraindicated for CKD cats.


Use of bisphosphonates to treat severe idiopathic hypercalcaemia in a young Ragdoll cat (2011) Whitney JL, Barrs VR, Wilkinson MR, Briscoe KA & Beatty JA Journal of Feline Medicine & Surgery 13(2) pp129-34 discusses the case of a young cat with idiopathic hypercalcaemia who became clinically normal following the use of bisphosphonate drugs. Idiopathic means no obvious cause could be found.


Low Calcium Levels

Low calcium levels are sometimes seen in CKD cats. The simplest treatment is a calcium supplement such as calcium carbonate (Tums). If this type of treatment is used, calcium blood levels should be checked regularly because the opposite problem of high calcium may result.


Calcitriol (see below) is also used in some cases because it tends to raise calcium levels.


Elevated PTH and Secondary Hyperparathyroidism


Untreated elevated parathyroid hormone (PTH) levels may eventually lead to a condition called secondary hyperparathyroidism. These are the treatments which may be used:

Phosphorus Control

The first line of attack to reduce the risks of secondary hyperparathyroidism is to feed your CKD cat a diet low in phosphorus. If this is not sufficient, you should also use phosphorus binders.


For many cats, phosphorus control will be enough to avoid secondary hyperparathyroidism - in one study, Survival of cats with naturally occurring chronic renal failure: effect of dietary management (2000) Elliott J, Rawlings JM, Markwell PJ, Barber PJ Journal of Small Animal Practice 41(6) pp235-242, a low phosphorus diet, with added binders where necessary, prevented the rise in parathyroid hormone levels seen in the control cats who were not restricted as regards phosphorus. In fact, the cats who ate reduced phosphorus food or food with added phosphorus binders lived more than twice as long as those who did not.


Phosphorus control is such an important topic for CKD cats that there is a page devoted to the subject here.



Famotidine (trade name is Pepcid AC) is commonly used in CKD cats to block the production of excess stomach acid. As a side effect it may reduce PTH levels in CKD patients. Famotidine reduces serum parathyroid hormone levels in uremic patients (1991) Arik N, Arinsoy T, Sayín M, Taşdemir I, Yasavul U, Turgan C, Caglar S Nephron 59(2) p333 explains more about this.


I would not use famotidine to treat elevated PTH levels only, but if you are using it anyway to control excess stomach acid, you may possibly see a reduction in PTH levels as well.



What is Calcitriol?

Calcitriol, or 1,25 dihydroxycholecalciferol, is the active form of Vitamin D. Despite its confusing name, it is not the same thing as Vitamin D, and is actually a hormone. It plays an important part in regulating phosphorus and calcium levels in the body. Calcitriol production tends to fall in CKD, which is a factor in the development of secondary hyperparathyroidism, so human patients with secondary hyperparathyroidism are sometimes given additional calcitriol, and some people do the same for their CKD cats.


Why the Use of Calcitriol in Cats is Controversial

The use of calcitriol in cats is somewhat controversial, in part because no studies clearly show that it is effective for cats (although it does appear to be effective for dogs).


The willingness or otherwise of US vets to use calcitriol may depend in part upon which vet school they attended. Initial studies at Cornell University College of Veterinary Medicine and later studies at the University of Georgia College of Veterinary Medicine found that using calcitriol caused calcium levels to rise. However, Ohio State University College of Veterinary Medicine has also conducted research into the use of calcitriol in CKD cats, and it believes that the risk of hypercalcaemia is dosage dependent and that the risk can be managed.


Calcitriol advocates believe that it is possible to have normal phosphorus levels and still have secondary hyperparathyroidism. They believe not only that the use of calcitriol is therefore essential, but also that calcitriol may help control uraemia and even slow the progression of CKD. Vets in the opposing camp believe that you can control PTH levels and prevent or postpone the development of secondary hyperparathyroidism by controlling phosphorus levels. They also are concerned about the potential for calcitriol supplementation to cause elevated calcium levels. In Renal disease (2006) Dr D Polzin states "the decision to use calcitriol must be made with caution because hypercalcemia is a potentially serious complication. Sustained calcitriol-induced hypercalcemia will likely result in reversible or irreversible reduction in GFR." GFR is a measure of kidney function.


What do I think? I tend to perch rather uncomfortably on the fence. Controlling secondary hyperparathyroidism is certainly important. Calcitriol may help with secondary hyperparathyroidism, and it does seem to help some cats feel better generally. I don't think using it is a bad idea if you follow the dosage and monitoring guidelines and stop it promptly if high calcium levels do result. On the other hand, its advocates recommend starting it really early in order to prevent secondary hyperparathyroidism and I'm not aware of any evidence for this being effective. The International Renal Interest Society (2015) states that calcitriol might be helpful for dogs in IRIS Stage 3 but that "the beneficial effects of ultra low dose calcitriol have not been established in cats."


Overall, I would be happy to consider using calcitriol, but I do not consider it essential. If you can find it, and can afford it and the accompanying testing schedule, and your cat seems to tolerate it, then you could give it a go. If you do want to try it, Dr Larry Nagode of Ohio State University College of Veterinary Medicine advises starting it when creatinine reaches 2 US mg/dl or 175 mmol/L international, assuming pre- and post-renal causes of the elevated creatinine have been ruled out. However, if you are unable to obtain it or cannot afford it, or if it does not seem to agree with your cat, I would give it a miss.


The US National Library of Medicine explains more about calcitriol.


Relationships of calcitriol to renal disease (2012) is a webinar by Dr Larry Nagode.


Calcitriol, calcidiol, parathyroid hormone, and fibroblast growth factor-23 interactions in chronic kidney disease (2013) de Brito Galvao JF, Nagode LA, Schenck PA & Chew DJ Journal of Veterinary Emergency and Critical Care 23(2) pp 134–162 discusses calcitriol (in very technical terms).


Considerations Before Starting Calcitriol

Calcitriol will not work for cats whose phosphorus levels are already above US: 8 mg/dl (international 2.6 mmol/L), and is of limited value for cats with phosphorus levels above US: 6 mg/dl (1.9 mmol/L).


On the other hand, it can still cause ionised calcium levels to increase. Prolonging life and kidney function (2007) a paper presented to the 32nd World Small Animal Veterinary Association World Congress by Dr D Chew, explains more about these issues. Therefore, if your cat's phosphorus levels are above 6 (US) or 1.9 (international), you must first take all possible steps to control phosphorus levels (see Phosphorus for information on how to do this).


You also need to consider your cat's calcium levels. If your cat's calcium level x phosphorus level is over 60-70 in US values or over 5 in international values, your cat is at risk of soft tissue mineralisation (see Diagnosis), and since calcitriol may raise calcium levels, you should not use it until this level has been reduced. In Chronic kidney disease in dogs and cats: the pivotal role of phosphorus control (2011) A Presentation to the 63rd CVMA Convention, Dr D Chew states: "Calcitriol should not be administered until hyperphosphatemia has been controlled. If the Ca X P solubility product exceeds 60-70, calcitriol should be avoided because of the risk of soft-tissue mineralization." Dr Chew also mentions that calcitriol will be less effective if your cat's ionised calcium level is low.


Calcitriol Dosage

The usual dose recommended for cats by Ohio State University College of Veterinary Medicine is 2.5-3.5 ng per kg of bodyweight orally each day. Note this is nanograms, not milligrams. This is a tiny amount, so in practice you must use compounded calcitriol in order to obtain cat-sized doses (see below for sources). 


Comparison of the effects of daily and intermittent-dose calcitriol on serum parathyroid hormone and ionized calcium concentrations in normal cats and cats with chronic kidney failure (2006) Hostutler RA, DiBartola SP, Chew DJ, Nagode LA, Schenck PA, Rajala-Schultz PJ, Drost WT Journal of Veterinary Internal Medicine 20(6) pp1307-13 found that intermittent dosing did not seem to make any difference in terms of effectiveness. Therefore most people these days use an intermittent dosing schedule, giving calcitriol every 3.5 days, in which case the dose is 8.75 - 12.25 ng twice a week. Dosing in this way is not only as effective as daily dosing, it can also work out cheaper, and may reduce the risk of hypercalcaemia. This timing must be exact, i.e. every 3.5 days rather than every three days or every four days, so many people use a schedule of giving it on Wednesday evenings and Sunday mornings, at a time to suit (e.g. 10 a.m. Sunday and 10 p.m. Wednesday).


A kg is 2.2 lbs, so if your cat weighs 10lb, 10lbs divided by 2.2 is 4.55kg. If you are giving the 2.5 ng starting dosage of calcitriol to this cat, you would therefore give 11.4 ng a day. If you are giving it every 3.5 days, you would give 38.8 ng (rounded up to 39 ng).


Calcitriol is given on an empty stomach, in either oil-filled capsules or oily liquid form. The amounts needed are tiny, so the treatment has to be compounded to the correct dosage for a cat. If you are using a liquid formulation, be sure to give it into the side of the mouth, not into the front. You should avoid giving any supplements etc. containing calcium on the days when you give calcitriol. Do not store liquid calcitriol in the fridge. Do not shake it.


It is usual to start with the lowest dose, and to check PTH, calcium and phosphorus levels after 10-14 days. You should check at least 24 hours after giving a dose of calcitriol. It is not normally necessary to fast your cat before testing, but I would not check immediately after eating - aim to keep food away from your cat for about four hours before the blood is taken.


Your goal is to reduce PTH levels, whilst keeping phosphorus levels low and calcium levels within normal range. If the levels are acceptable, you should check again after about a month. If they still are out of range, adjust the dose and check again after 10-14 days. Once you have determined the correct dose, you should continue to check PTH, calcium and phosphorus levels every 4-8 weeks.


Benefits of calcitriol therapy and serum phosphorus control in cats and dogs with chronic renal failure. Both are essential to prevent or suppress toxic hyperparathyroidism (1996) Nagode LA, Chew DJ, Podell M Veterinary Clinics of North American Small Animal Practice 26 pp1293-1330 discusses the important of controlling secondary hyperparathyroidism in cats and the role of calcitriol in doing so.


In Renal Disease (2006) Polzin DJ Delaware Valley Academy of Veterinary Medicine (2006) Dr Polzin states "Nagode and colleagues have suggested that normalization of PTH levels using calcitriol therapy may provide clinical benefits that cannot be achieved by phosphorus restriction alone including amelioration of many clinical signs associated with CKD. We have been unable to completely substantiate these claims, but did find that calcitriol therapy significantly prolonged survival in dogs with stages 3 and 4 CKD."  He further states "A recommendation for or against routine use of calcitriol awaits results of properly designed controlled clinical trials."


Mar Vista Vet has more information on calcitriol.


Calcitriol Group - a group devoted to discussing the use of calcitriol in cats and dogs.


Calcitriol Interactions and Side Effects

The US National Library of Medicine National Institutes of Health lists possible side effects, including weakness, vomiting, constipation, and increased urination, especially at night. Some cats seem to feel a little off colour on the days when they are given calcitriol. In such cases, the twice weekly dosing schedule seems to work better.


Using calcitriol may increase your cat's phosphorus levels somewhat. Keep an eye on this, and increase your phosphorus binder dose as necessary.


Calcitriol may also cause your cat's calcium levels to rise too far. If this happens, you will probably have to stop using calcitriol, after which calcium levels should reduce within a few days or so.


According to Plumb's Veterinary Drug Handbook, corticosteroids may cancel out the effects of calcitriol. This is because calcitriol increases calcium absorption, whereas corticosteroids inhibit calcium absorption.


Calcitriol Sources

Since cats need only tiny doses of calcitriol, you usually have to have it compounded. It comes in an oily suspension, and the compounding pharmacy normally needs to dilute this further with more oil in order to create cat-sized doses. The end product is supplied in either a liquid, oily form, or is hardened and supplied in capsules.



Calcitriol is not too expensive in the USA. If you buy six months supply at a time, it works out at about US$10 a month. If you do buy six months' supply at a time, make sure its shelf life is for six months or longer.


You do need an experienced compounder, because the doses are so tiny that mistakes may happen. These compounded pharmacies are often recommended in the USA:

  • Wells Pharmacy Network. Apparently Wells took over the business of Francks Pharmacy, which closed in July 2012 following problems with quality control. I do not know anyone who has used them yet, though most people were satisfied with the calcitriol they got from Francks.

  • Thriving Pets If you enter the word "tanya" (without the ") in the promotional code box, you will receive a 10% discount on orders over US$55.

  • Triad Compounding

  • US Compounding


The use of calcitriol is difficult in the UK, because it is very hard to find a pharmacy which is authorised to compound medications for veterinary use into cat-sized dosages. Some people use human calcitriol but I'm not too clear how they manage to obtain cat-sized doses from it.


If you can find a vet with a US licence who can write a prescription for calcitriol, Thriving Pets will fill it for you. If you enter the word "tanya" (without the ") in the promotional code box, you will receive a 10% discount on orders over US$55.


If you are unable to obtain calcitriol, please do not be too despondent. It is not an essential treatment for most CKD cats. If you focus on phosphorus control, for many cats this will be sufficient to avoid secondary hyperparathyroidism, except possibly for very end stage cats. 


Cinacalcet (Sensipar)

Sensipar (cinacalcet HCl) belongs to a family of drugs known as calcimimetics and was approved by the USA Food and Drug Administration in March 2004 for the treatment of secondary hyperparathyroidism in humans. It is also used to treat hypercalcaemia caused by parathyroid cancer.


Sensipar works by lowering calcium levels, so should not be used in patients with low calcium levels. It is very expensive. I have only heard of a couple of people who have used it in cats, and never heard back to see how they got on with it..


Elevated PTH levels can contribute to anaemia. Improved parathyroid hormone control by cinacalcet is associated with reduction on darbepoetin requirement in patients with end-stage renal disease (2011) Battistella M, Richardson RM, Bargman JM & Chan CT Clinical Nephrology 76(2) pp99-103 found that cinacalcet meant less treatment for anaemia was required in human patients. I don't think this is unique to cinacalcet, but rather because of the reduction in PTH levels.


Amgen - prescribing information from the manufacturers of Sensipar.

The US National Library of Medicine explains more about cinacalcet.

Cinacalcet hydrochloride (Sensipar) (2005) Poon G Baylor University Medical Center Proceedings 18(2) pp182-184 gives an overview of cinacalcet.





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