Hypercalcaemia in Cats: When Elevated Calcium Is a Red Flag
Hypercalcaemia — abnormally elevated calcium in the blood — is a finding that demands attention in cats. While calcium is essential for bone structure, muscle function, nerve transmission, and blood clotting, too much of it in circulation disrupts multiple organ systems simultaneously. In cats, persistent hypercalcaemia is frequently a marker of serious underlying disease, and identifying the cause is as important as managing the elevated calcium itself.
What Calcium Measurement Tells Us
Most routine blood panels measure total serum calcium, which includes calcium bound to proteins (primarily albumin), calcium complexed with anions, and ionised calcium — the biologically active fraction. In cats, measurement of ionised calcium is considered more clinically informative than total calcium, particularly because low albumin levels (common in sick cats) can falsely lower total calcium readings and mask true hypercalcaemia. Conversely, some cats with elevated total calcium have normal ionised calcium, and the clinical significance is different.
When your vet identifies elevated calcium on a blood test, the next step is usually to confirm the finding with ionised calcium measurement if not already done, and then pursue a systematic investigation into the underlying cause.
Common Causes in Cats
Idiopathic hypercalcaemia
Idiopathic hypercalcaemia (IHC) is the most commonly identified cause of persistent hypercalcaemia in cats in many referral populations, meaning no underlying cause can be found despite thorough investigation. It is thought to involve dysregulation of calcium metabolism at the intestinal level, possibly related to abnormal vitamin D metabolite activity. Middle-aged to older cats are most commonly affected, and many are maintained on high-fibre diets as a management strategy, with some evidence that this approach reduces intestinal calcium absorption. The long-term consequence of untreated IHC is kidney disease, because persistently elevated calcium is toxic to renal tubular cells.
Malignancy
Cancer is a critically important cause of hypercalcaemia in cats and one that must be ruled out before a diagnosis of idiopathic disease is accepted. Hypercalcaemia of malignancy occurs through two main mechanisms: tumours may produce parathyroid hormone-related protein (PTHrP), which mimics the action of parathyroid hormone and drives calcium out of bone and into the blood; or bone metastases may directly release calcium as they destroy bone tissue.
Lymphoma is the malignancy most commonly associated with hypercalcaemia in cats. Any cat with hypercalcaemia and signs suggestive of lymphoma — weight loss, gastrointestinal signs, lymph node enlargement — should be investigated for this before other diagnoses are considered. Other tumours including thymoma, squamous cell carcinoma, and multiple myeloma can also cause elevated calcium.
Chronic kidney disease
The relationship between calcium and kidney disease in cats is bidirectional and complex. Chronic kidney disease (CKD) can cause hypercalcaemia, and hypercalcaemia causes CKD. In cats with CKD, calcium accumulation in renal tissue (nephrocalcinosis) is a recognised complication that accelerates functional decline. Distinguishing whether hypercalcaemia is the cause or consequence of renal disease in a given cat requires careful evaluation.
Primary hyperparathyroidism
The parathyroid glands, tiny structures embedded near the thyroid glands in the neck, regulate calcium through secretion of parathyroid hormone (PTH). A benign parathyroid adenoma can cause autonomous overproduction of PTH, resulting in hypercalcaemia. This condition is less common in cats than in dogs but does occur. Measurement of PTH alongside calcium helps to identify this cause — in primary hyperparathyroidism, PTH is inappropriately elevated relative to the high calcium level.
Other causes
Additional causes include hypervitaminosis D (from dietary supplements or certain rodenticides), granulomatous diseases such as fungal infections or FIP (feline infectious peritonitis), and hyperthyroidism — though hyperthyroidism more commonly causes mild, incidental elevation. Some cats hospitalised on intravenous fluids with calcium content may show transient elevation.
Clinical Signs of Hypercalcaemia
The clinical signs depend on how high the calcium is and how rapidly it has risen. Mild or slowly developing hypercalcaemia may cause few or no obvious symptoms. As levels rise or persist, signs emerge that reflect the effect of excess calcium on multiple systems:
- Increased thirst and urination — calcium interferes with the kidney's ability to concentrate urine
- Reduced appetite and weight loss
- Vomiting and constipation
- Lethargy and muscle weakness
- Twitching or in severe cases, seizures
- Cardiac arrhythmias in extreme cases
The mnemonic "bones, stones, groans and moans" — familiar from human medicine — captures the main organ systems affected: bones (demineralisation), kidneys (stones and failure), gastrointestinal tract (nausea, vomiting, constipation), and neurological and cardiovascular systems.
Diagnostic Investigation
A systematic approach to the hypercalcaemic cat includes full biochemistry and haematology, urinalysis, measurement of ionised calcium, PTH and PTHrP concentrations, vitamin D metabolite levels where available, thoracic radiographs, abdominal ultrasound, and lymph node assessment. Bone marrow evaluation may be indicated if multiple myeloma is suspected. The aim is to work through the differential diagnosis list methodically before attributing the finding to idiopathic disease.
Management
Treatment is directed at the underlying cause wherever one is identified. In cases of malignancy, treating the cancer — whether with chemotherapy for lymphoma or surgery for a parathyroid adenoma — typically resolves the hypercalcaemia. For primary hyperparathyroidism, surgical removal of the affected parathyroid gland is curative in most cases.
For idiopathic hypercalcaemia, a high-fibre diet is the first-line approach, often combined with ensuring adequate hydration. Bisphosphonate medications, which reduce bone calcium release, are used in refractory cases. Corticosteroids can lower calcium and are sometimes used in the short term, though they complicate the diagnostic picture if lymphoma has not been fully excluded and should be used with caution.
Monitoring renal function throughout is essential, because the kidneys bear the brunt of sustained hypercalcaemia. Regular rechecks of serum calcium, renal parameters, and urinalysis allow treatment to be adjusted as the clinical picture evolves.