Parathyroid Disease in Cats: Calcium Imbalance and Its Effects on Health

Your cat's blood test came back with one line highlighted: calcium, high. Nothing else looked dramatic, and your cat seems mostly fine. That single number is worth taking seriously, because calcium is one of the few things the feline body cannot afford to get wrong.

Key Takeaways

• Calcium in your cat's blood is controlled by two tiny parathyroid glands sitting beside the thyroid in the neck, and by vitamin D and calcitonin working alongside them.
• Too much calcium (hypercalcemia) is far more common in cats than too little, and the top causes are kidney failure, cancer and a strange feline-only syndrome called idiopathic hypercalcemia.
• Too little calcium (hypocalcemia) shows up as twitching, tremors and seizures, and it is most often linked to kidney disease, to nursing queens, or to surgery that damaged the parathyroid glands.
• Primary parathyroid tumours, the classic cause in humans and dogs, are relatively rare in cats.
• Many cats with abnormal calcium show no obvious symptoms at all, which is why a routine annual blood panel after age seven is the single most useful thing you can do.
• Most calcium disorders are managed, not cured, and cats caught before kidney or bladder damage usually do very well.

What Are the Parathyroid Glands and What Do They Actually Do?

The parathyroid glands are small hormone factories that sit on either side of the thyroid gland in your cat's neck. They are tiny, usually only a few millimetres across, and they have exactly one job: keep blood calcium steady.

They do it by releasing parathyroid hormone (PTH). According to the Merck Veterinary Manual, PTH controls calcium by moving it into and out of bone, by changing how much calcium the kidneys hold on to, and by influencing how much calcium is absorbed from food in the gut.

Think of it like the water tank on an Indian rooftop. The tank has a float valve. When the water level drops, the valve opens and refills it. When it is full, the valve shuts. PTH is that float valve, and blood calcium is the water level. When the valve sticks open or breaks off entirely, the whole system floods or runs dry.

Two other players share the job:

• Vitamin D acts more like a hormone than a vitamin. Merck notes that unlike horses and humans, cats cannot make enough vitamin D in their skin from sunlight, so they depend almost entirely on what they eat. Your cat sunbathing on the balcony is enjoying the warmth, not manufacturing vitamin D.
• Calcitonin, released by the thyroid gland, is the brake. When calcium climbs too high, calcitonin steps in to pull it back down.

Why Calcium Matters So Much in a Cat's Body

Most people think of calcium as a bone mineral. It is, but that is the least interesting part of the story.

Calcium is essential for muscle contraction, blood clotting, enzyme activity, nerve signalling and hormone release. Every time your cat's heart beats, every time a nerve fires, calcium is involved. This is why the body guards the level so tightly and why a disturbance shows up as neurological and muscular signs rather than as anything bone related.

There is one more technical point that matters when you read a lab report. Calcium circulates in three forms: bound to proteins, complexed with other substances, and ionized calcium, which is the biologically active form. Roughly half of circulating calcium is ionized. A standard biochemistry panel reports total calcium, which can be misleading in a sick cat. Ionized calcium is the number that tells the real story, and it needs a specific test.

What Is Parathyroid Disease in Cats?

Parathyroid disease is any condition where the calcium control system fails. It splits into two directions:

Here is the important nuance. Only a small fraction of feline calcium problems come from a diseased parathyroid gland itself. Primary hyperparathyroidism, where a benign tumour on the gland pumps out excess PTH, is relatively rare in cats. Far more often, the parathyroid glands are healthy and are reacting correctly to a problem somewhere else, usually the kidneys.

That distinction shapes everything about treatment. You do not treat the gland. You find and treat what is upsetting it.

Hypercalcemia: When Calcium Runs Too High

Hypercalcemia means abnormally high blood calcium. Merck describes it plainly: excess calcium in the blood is harmful to all tissues, but especially to the kidneys, nervous system, heart and blood vessels. Extremely high levels can be life threatening.

How sick a cat gets depends on three things: how high the calcium is, how fast it rose, and how long it has stayed there. A cat whose calcium crept up slowly over a year may look completely normal. A cat whose calcium spiked in a week may collapse.

The most common signs are increased thirst and urination, followed by reduced appetite, vomiting, constipation, weakness, depression, muscle twitching and seizures.

One thing worth flagging for cat parents specifically: increased drinking and urinating is the textbook sign in dogs, but in cats it is often absent. Studies of feline idiopathic hypercalcemia have found that many affected cats never show increased thirst at all. Do not use "she's drinking normally" as reassurance.

The hidden damage: bladder and kidney stones

When calcium runs high in the blood, more of it gets dumped into the urine. Concentrated cat urine plus excess calcium is a recipe for calcium oxalate stones, the kind that are hard, sharp and cannot be dissolved with diet.

This matters enormously. In a large study of 238 cats with elevated ionized calcium published in the Journal of Veterinary Internal Medicine, urolithiasis (stones in the urinary tract) was found in half of the imaged cats with idiopathic hypercalcemia, and in over 80 percent of those with acute kidney injury.

So a cat presenting with straining in the litter box may actually be presenting with a calcium problem. If that sounds familiar, our guide on what it means when your cat strains in the litter box or pees blood walks through the emergency signs in detail.

What Causes High Calcium in Cats?

Merck lists the causes and mechanisms clearly. Here they are in the order that matters for cats:

There is one more cause that Indian pet parents should know about, and it is entirely preventable: vitamin D toxicity. Cats depend on dietary vitamin D and have no way to switch off absorption. Over-supplementing with human calcium or vitamin D tablets, or a cat getting into rodenticide containing cholecalciferol, can push calcium to dangerous levels. Never give your cat a human supplement without your vet telling you to.

Idiopathic Hypercalcemia: The Feline-Only Puzzle

Idiopathic simply means "we do not know why." Idiopathic hypercalcemia is one of the more frustrating diagnoses in feline medicine because it is defined by what it is not.

What is known:

• It appears to be unique to cats. Dogs do not get it as a distinct syndrome.
• It affects cats across a wide age range, generally between 2 and 13 years, with no sex preference.
• Long-haired cats appear to be over-represented.
• It usually produces mild to moderate elevation in calcium. Severe spikes are unusual.
• Signs are vague. Some cats look entirely normal. Others show weight loss, reduced appetite, occasional vomiting or constipation, and these signs come and go.

It is a diagnosis of exclusion. Your vet must first rule out kidney disease, cancer, granulomatous disease and parathyroid tumours. Only when everything else comes back clean does the label get applied.

That process costs money and takes time, and many cat parents get impatient with it. It is worth understanding why it happens: the treatment for cancer-driven hypercalcemia is completely different from the treatment for idiopathic hypercalcemia. Skipping the workup means guessing.

Hypocalcemia: When Calcium Drops Too Low

Hypocalcemia is abnormally low blood calcium, and it produces a very different picture. Merck describes the classic signs as twitching, muscle tremors and seizures.

The causes:

Hypoparathyroidism. The parathyroid glands stop producing enough PTH. Blood calcium falls, phosphate rises. Merck notes this is rare in cats, and when it occurs it is usually a consequence of surgery, specifically the removal of parathyroid tissue during treatment of hyperthyroidism or a parathyroid tumour. Signs include muscle tremors, twitching, muscle contraction and generalised convulsions.

Chronic kidney failure. Merck describes this as probably the most common cause of hypocalcemia. The good news is that this form usually does not produce dramatic nervous system signs the way hypoparathyroidism does. Management centres on lowering blood phosphate through diet and medication.

Eclampsia (puerperal tetany). This occurs in nursing queens, where calcium leaving the body in milk outpaces what is coming in. It is an emergency. Treatment is intravenous calcium, and weaning the litter if possible.

Chelating agents. Substances that bind calcium can drop levels sharply. The most notorious is ethylene glycol, the sweet-tasting compound in antifreeze and in some radiator coolants. Cats are extraordinarily sensitive to it.

Human enemas. Merck specifically warns that over-the-counter phosphate enemas made for humans can cause severe electrolyte and fluid abnormalities in cats, leading to twitching, tremors and shock. These effects are life threatening. If your cat is constipated, do not reach for a human enema from the chemist. Call your vet.

Other contributors include low blood protein, poor intestinal absorption and pancreatitis.

What Are the Symptoms of Calcium Imbalance in Cats?

Because this is the question most people actually type into Google, here is the direct answer first.

High calcium in cats most often causes increased thirst and urination, reduced appetite, vomiting, constipation, weakness, lethargy, muscle twitching and in severe cases seizures. Low calcium causes muscle tremors, twitching, facial rubbing, stiff gait, restlessness and seizures. Many cats with mild calcium abnormalities show no symptoms at all.

Now the expanded version, organised by what you would actually notice at home.

Changes in the litter box. Larger or more frequent urine clumps. Straining. Blood. Or the opposite, hard dry stools and visible effort to defecate. Constipation is a genuinely common sign of high calcium and is easy to dismiss as a hairball problem.

Changes at the food bowl. Reduced interest in food, slow gradual weight loss, occasional vomiting that does not fit a pattern. In idiopathic hypercalcemia these signs typically wax and wane, which is exactly why they get ignored.

Changes in movement and behaviour. Weakness, reluctance to jump, sleeping more, a flat and withdrawn mood. If your cat has become notably sluggish, our article on how to prevent lethargy in your cat covers the broader differential.

Neurological signs. Twitching around the face and ears, tremors, rippling skin, disorientation, or seizures. These are urgent. A cat having a seizure needs a vet immediately, not tomorrow morning.

Nothing at all. This is the most important entry on the list. A meaningful proportion of cats with abnormal calcium are picked up purely on routine bloodwork with no clinical complaint whatsoever.

How Vets Diagnose Parathyroid and Calcium Disorders

Merck is direct about the reality here: the cause of hypercalcemia may not be apparent and may require a number of tests, including routine and specialised blood tests, diagnostic imaging such as radiographs and ultrasonography, and biopsies.

Here is what that sequence usually looks like.

Step 1: Full biochemistry panel and complete blood count. This is where total calcium usually first appears as abnormal. It also gives kidney values, phosphorus and protein levels.

Step 2: Ionized calcium. The confirmatory test. Total calcium can be falsely high or falsely low depending on protein levels and acid-base status. Ionized calcium is the biologically active fraction and it is what a diagnosis should rest on. Not every small clinic runs this in-house, so expect a sample to be sent out.

Step 3: PTH and PTHrP. Measuring parathyroid hormone alongside calcium tells your vet whether the gland is behaving appropriately. High calcium with high PTH points to a parathyroid problem. High calcium with suppressed PTH points elsewhere, often to cancer. Parathyroid hormone-related protein (PTHrP) is the marker for cancer-driven hypercalcemia.

Step 4: Vitamin D metabolites, where toxicity or granulomatous disease is suspected.

Step 5: Imaging. Neck ultrasound to look at the parathyroid glands, which are normally only 2 to 4 millimetres and genuinely hard to see. Abdominal ultrasound and radiographs to look for tumours, kidney changes, and stones in the bladder or kidneys. Chest radiographs to screen for lymphoma.

Step 6: Urinalysis. Checks concentrating ability and looks for crystals.

Step 7: Biopsy or aspirate, where a mass is found.

A practical note for Indian cat parents. Ionized calcium and PTH assays are not available everywhere. In metros like Mumbai, Delhi, Bangalore, Chennai, Pune and Hyderabad you will find referral practices and labs that run them. In smaller cities, your vet may need to ship samples, which adds days. Ask upfront whether the sample needs special handling, and do not let the delay talk you out of the test.

Treatment: What Actually Works

Merck's framing is worth repeating: hypercalcemia is treated by identifying and treating the condition causing it. There is no single drug for "high calcium."

Supportive treatment to bring calcium down

For a cat that is unwell, the first goal is to lower calcium while the workup continues. Merck lists intravenous fluids, diuretics and glucocorticoids as the standard supportive measures, with additional drugs available. Fluids dilute the blood and push calcium out through the kidneys. Diuretics increase that excretion.

A word of caution on steroids: glucocorticoids can mask or partially treat lymphoma, which makes a cancer diagnosis harder afterwards. Good vets usually want the diagnostic samples collected before steroids start. If your vet asks to wait, that is why.

Treating the specific cause

• Primary hyperparathyroidism: removal or destruction of the tumour.
• Cancer-related: treatment depends entirely on the cancer type.
• Chronic kidney failure: supportive care, phosphate control and a renal diet.
• Hyperthyroidism: surgery or radioiodine to treat the thyroid tumour. Merck notes calcium may normalise afterwards but should still be monitored.
• Granulomatous disease: treat the underlying infection.
• Idiopathic hypercalcemia: diet change first, and possibly medication such as prednisone or bisphosphonates.

Bisphosphonates deserve a mention because they are increasingly used in cats. They were developed for osteoporosis in humans and work by driving calcium back into bone. They can be effective for long-term control of idiopathic hypercalcemia, but they need careful dosing and monitoring.

Treating low calcium

If a cat is seizing or in muscle spasm from hypocalcemia, Merck is unambiguous: immediate intravenous calcium is needed. This is a hospital procedure, given slowly with cardiac monitoring, never something to attempt at home. For long-term management of hypoparathyroidism, oral calcium supplements are prescribed, usually together with vitamin D.

Diet and Long-Term Management at Home

Diet is the first line of management for idiopathic hypercalcemia and a cornerstone of care for kidney-linked calcium problems. It is also the part you control.

Wet food over dry, wherever possible. Cats evolved in arid environments and have a weak thirst drive. More water in the diet means more dilute urine, and dilute urine is the single best defence against calcium oxalate stones. If your cat is on a renal or urinary prescription diet, adding the wet version of the same line is usually the easiest upgrade.

Prescription therapeutic diets, chosen by diagnosis. For a cat with chronic kidney disease driving the calcium problem, a renal diet with controlled phosphorus is standard. Royal Canin Renal Cat Dry Food is formulated for exactly this, with restricted phosphorus and omega-3 fatty acids. For cats who have gone off their food, which is extremely common in kidney disease, Royal Canin Renal Cat Tuna Jelly in pouch format gives the same renal support with far more moisture and much stronger palatability. Where the main risk is stone formation, RC Urinary Care Cat Dry Food is designed to reduce the likelihood of crystal and stone formation in the urinary tract. All of these are therapeutic diets and should be started on your vet's instruction, not on your own reading.

Stop all calcium and vitamin D supplements unless prescribed. This includes human calcium tablets, cod liver oil and general multivitamins. In a cat with high calcium, these actively worsen the problem.

Water everywhere. Multiple bowls in different rooms, away from the litter tray and the food bowl. Many cats drink noticeably more from a running fountain. In Indian summers, when ambient temperature pushes water loss up sharply, this matters even more.

Monitoring schedule. Expect rechecks. Cats with stable, uncomplicated hypercalcemia are typically rechecked every few months. Cats with stones or existing kidney damage need closer follow-up. Skipping rechecks because the cat "looks fine" is the most common way these cases go wrong, because looking fine is the default state of this disease.

What Indian Cat Parents Should Watch For

A few points specific to where you live.

Routine senior bloodwork is not standard practice yet, and it should be. Most calcium abnormalities in cats are found incidentally. If your cat is over seven, an annual full biochemistry panel plus urinalysis is the intervention with the highest return. If your cat is over ten, make it twice a year.

Do not self-medicate constipation. Constipation is common in Indian households where cats eat mainly dry food in a hot climate, and it is also a symptom of hypercalcemia. Human phosphate enemas can be fatal in cats. Human laxatives are not safe either. If your cat has not passed stool in over 48 hours, that is a vet visit.

Homemade diets need real formulation. Cats fed on leftover rice, curd and fish scraps are at genuine risk of calcium, phosphorus and vitamin D imbalance, and a growing kitten on such a diet can develop skeletal disease. If you feed home-cooked food, get the recipe balanced by a veterinary nutritionist.

Ethylene glycol exposure is real. Radiator coolant in society parking areas is a genuine hazard for cats who roam. It tastes sweet, and a very small amount is lethal.

Ask for ionized calcium specifically. If a routine panel shows high total calcium, the follow-up test you want named is ionized calcium. Knowing the term makes the conversation faster.

Prognosis: What Happens Long Term

The honest answer is that it depends entirely on the cause.

Cats with a benign parathyroid tumour that is successfully removed generally do very well, though calcium can crash after surgery and needs close monitoring in the days afterwards.

Cats with idiopathic hypercalcemia usually live normal lives with lifelong management. The dividing line is organ damage. Cats diagnosed before bladder stones or kidney damage develop generally maintain good quality of life. Cats who already have stones or reduced kidney function have a more guarded outlook, though many are managed successfully for years.

Cats whose hypercalcemia comes from cancer face the toughest road. Merck notes that the presence of hypercalcemia in these cases is often associated with a poorer outlook and shorter survival time.

Cats with kidney-linked calcium problems follow the trajectory of their kidney disease, which is manageable and often slow-moving when caught early.

The pattern across all four is the same. Early detection changes the outcome more than any specific drug does. That is the whole argument for annual bloodwork.

FAQ

Is parathyroid disease common in cats?
True parathyroid disease is uncommon in cats. Merck describes primary hyperparathyroidism as relatively rare in this species, and hypoparathyroidism as rare and usually surgical in origin. What is common is abnormal blood calcium driven by something else, most often chronic kidney disease, cancer, or idiopathic hypercalcemia. The parathyroid glands themselves are usually innocent bystanders responding correctly to a problem elsewhere.

Can high calcium in cats be cured?
Sometimes. If the cause is a single benign parathyroid tumour, removing it can resolve the problem completely. If the cause is idiopathic hypercalcemia, it is managed rather than cured, with lifelong diet control and often medication. If the cause is chronic kidney disease or cancer, calcium control follows management of the underlying condition. The realistic goal for most cats is stable calcium and protected kidneys, not a permanent fix.

What are the first signs of calcium problems in cats?
Often there are none. When signs do appear, the earliest are usually subtle: slightly reduced appetite, gradual weight loss, occasional vomiting, constipation, or a cat who is simply less active than before. Increased thirst and urination is the classic textbook sign but is frequently absent in cats. Tremors, twitching or seizures point to low calcium rather than high, and are urgent.

Can diet alone fix high calcium in my cat?
For idiopathic hypercalcemia, a diet change is genuinely the recommended first step and works for a proportion of cats. Merck lists diet change, with medication such as prednisone or bisphosphonates as the next option if needed. For hypercalcemia caused by cancer, kidney failure or a parathyroid tumour, diet supports management but does not resolve the underlying disease. Never switch to a therapeutic diet without a diagnosis, because the wrong prescription diet can make things worse.

Is hypercalcemia in cats painful?
Not directly, in most cases. Cats with mild to moderate hypercalcemia usually seem uncomfortable rather than in pain, showing nausea, constipation and weakness. Pain enters the picture when complications develop, particularly calcium oxalate stones in the bladder or a stone lodged in the urethra, which is both extremely painful and a genuine emergency.

My cat's calcium is high but she seems totally normal. Should I still investigate?
Yes. Subclinical hypercalcemia is the norm rather than the exception in cats, and the damage to kidneys and the urinary tract accumulates silently. Confirming with an ionized calcium test and identifying the cause while your cat is well gives you far more treatment options than waiting until symptoms force the issue.

REFERENCES

1. Peterson, M.E., DVM, DACVIM-SAIM, Animal Endocrine Clinic, New York. Disorders of the Parathyroid Glands and of Calcium Metabolism in Cats. Merck Veterinary Manual, Pet Owner Version. https://www.merckvetmanual.com/cat-owners/hormonal-disorders-of-cats/disorders-of-the-parathyroid-glands-and-of-calcium-metabolism-in-cats
2. Ham, K., DVM, MS, DACVS, University of Florida. Overview of the Parathyroid Glands and Disorders of Calcium Regulation in Dogs and Cats. Merck Veterinary Manual. https://www.merckvetmanual.com/endocrine-system/the-parathyroid-glands-and-disorders-of-calcium-regulation-in-dogs-and-cats/overview-of-the-parathyroid-glands-and-disorders-of-calcium-regulation-in-dogs-and-cats
3. Hypercalcemia in Dogs and Cats. Merck Veterinary Manual, Endocrine System. https://www.merckvetmanual.com/endocrine-system/the-parathyroid-glands-and-disorders-of-calcium-regulation-in-dogs-and-cats/hypercalcemia-in-dogs-and-cats
4. Physiology and Calcium-Regulating Hormones in Dogs and Cats. Merck Veterinary Manual, Endocrine System. https://www.merckvetmanual.com/endocrine-system/the-parathyroid-glands-and-disorders-of-calcium-regulation-in-dogs-and-cats/physiology-and-calcium-regulating-hormones-in-dogs-and-cats
5. VCA Animal Hospitals. Hypercalcemia. https://vcahospitals.com/know-your-pet/hypercalcemia
6. Broughton, S.E., O'Neill, D.G., Syme, H.M., Geddes, R.F. Ionized hypercalcemia in 238 cats from a referral hospital population (2009-2019). Journal of Veterinary Internal Medicine, 2023. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9889682/
7. de Brito Galvao, J.F., Schenck, P.A., Chew, D.J. Update on Feline Ionized Hypercalcemia. Veterinary Clinics of North America: Small Animal Practice. https://pubmed.ncbi.nlm.nih.gov/27988049/