Serious severity
Hypercalcaemia (High Blood Calcium)
Raised blood calcium is rarely a disease in itself — it is a signal. Finding out what is driving it matters as much as bringing the level down.
⚠️ Common symptoms
Calcium in the blood is held within a narrow range, because so much depends on it: nerve signalling, muscle contraction, blood clotting, and the function of the heart and kidneys. When it drifts too high, the symptoms that follow are frustratingly vague — a cat that drinks more, eats less, seems tired, and becomes constipated. Owners rarely bring a cat in because of hypercalcaemia. It is usually found on a blood test run for some other reason, and the real work begins after it is found.
That is because hypercalcaemia is a finding, not a diagnosis, and the possible explanations behind it vary enormously in seriousness. Cats are unusual in having a well-recognised idiopathic form — a persistently raised calcium with no identifiable cause, diagnosed only after other possibilities have been ruled out. It is the most common form in cats and often runs a slow, manageable course, though it can lead to calcium oxalate bladder and kidney stones over time.
The possibility that has to be excluded first is cancer. Lymphoma is the classic culprit: some tumours secrete a substance that mimics parathyroid hormone, driving calcium out of bone and into the bloodstream. Chronic kidney disease is another common association, and the relationship runs in both directions, since prolonged hypercalcaemia can itself damage the kidneys by depositing calcium in the tissue. Primary hyperparathyroidism, in which a small tumour of the parathyroid gland overproduces the hormone that governs calcium, is uncommon in cats but eminently treatable with surgery.
Then there is the acute, toxic form, which behaves entirely differently. Cholecalciferol — vitamin D3 — is used in some rodenticides, and a cat that eats bait, or a poisoned rodent, absorbs a dose of vitamin D far beyond anything physiological. Calcium climbs steeply over a day or two, and calcium-phosphate complexes begin depositing in the kidneys, heart and blood vessels. Unlike the chronic forms, this is a genuine emergency, and it can also arise from vitamin D supplements or from psoriasis creams containing vitamin D analogues, which cats sometimes ingest by grooming a person's treated skin.
For owners, the takeaway is twofold. Investigate rather than assume: a raised calcium found on a routine test deserves a full work-up, because the difference between idiopathic hypercalcaemia and lymphoma is the difference between long-term management and a serious cancer diagnosis. And take vitamin D seriously as a household poison — bait boxes, supplement bottles and skin creams all belong out of reach, and any suspected exposure warrants a phone call to the vet the same day.
🔍 Causes
There are several distinct routes to a high calcium in cats. Idiopathic hypercalcaemia — genuinely unexplained, and the commonest form in cats — is diagnosed once everything else has been excluded. Cancer, particularly lymphoma, can raise calcium by releasing a hormone-like substance, a phenomenon known as hypercalcaemia of malignancy. Chronic kidney disease disturbs calcium and phosphorus handling and can push calcium either way. Cholecalciferol (vitamin D3) rodenticide poisoning causes a rapid, severe and dangerous rise, as can over-supplementation with vitamin D or psoriasis creams containing vitamin D analogues. Less commonly, primary hyperparathyroidism, granulomatous disease such as a fungal infection, or hypoadrenocorticism are responsible.
🛡 Prevention
Most causes cannot be prevented, but two important ones can. Never use cholecalciferol-based rodenticides in a home with a cat, and keep any rodent bait, vitamin D supplements and vitamin-D-containing skin creams completely inaccessible — cats can be poisoned by licking a treated human's skin. Beyond that, prevention is really early detection: routine blood screening for cats from middle age onwards, and particularly for cats with kidney disease, often finds a raised calcium before it has caused any harm.
💊 Treatment
Management has two parts: lowering the calcium and treating whatever is causing it. Vets typically begin with intravenous fluid therapy, which dilutes the blood and promotes calcium excretion through the kidneys, and may add other medications for severe or symptomatic cases. The lasting solution, though, depends on the cause — chemotherapy for lymphoma, surgery for a parathyroid tumour, decontamination and specific treatment for cholecalciferol rodenticide, or dietary and medical management for idiopathic hypercalcaemia. Because the causes are so different in outlook and treatment, diagnostic testing is not an optional extra here; it is the whole point.
⚕️ Medical disclaimer
- This information is educational only and is not a substitute for veterinary advice.
- Symptoms overlap between conditions — only a vet can diagnose accurately.
- Never withhold treatment or self-medicate based on information found online.
- When in doubt, call your vet. Early intervention almost always leads to better outcomes.