Primary hyperparathyroidism (relatively rare in cats)

Parathyroid glands are overactive (often due to a tumor) and secrete excessive parathyroid hormone

Removal or destruction of tumor

Hypercalcemia caused by cancer (malignant hypercalcemia); most commonly caused by lymphoma, squamous cell carcinoma, and multiple myeloma

Increased bone breakdown and resorption triggered by cancer cells causes higher levels of calcium in blood

Depends on specific type of cancer; the presence of hypercalcemia is often associated with a poorer outlook and a shorter survival time

Chronic kidney failure

Exact mechanism is unknown; most common cause of hyper-calcemia in cats

Supportive care

Hyperthyroidism (increased production of thyroid hormones)

Most cases are caused by a benign tumor of the thyroid gland that produces excess thyroid hormones. Those hormones can cause calcium levels to increase.

Surgery or radiation are often used to treat the tumor. Once that's been done, calcium levels may return to normal, but should be monitored.

Granulomatous disease (masses of white blood cells that can form because of certain fungal, bacterial, and parasitic infections)

Inflammation activates the type of white blood cells that can increase levels of active vitamin D

Treatment of underlying disease, for example, antifungal drugs or surgery

Idiopathic hypercalcemia of cats

Unknown cause; affects cats 2 to 13 years old (either sex)

Diet change and possibly medication (prednisone or bisphosphonates)