High Calcium in Cats: Normal Range, Causes & What the Chemistry Panel Value Means

1. Idiopathic Hypercalcemia (IHC) — Most Common in Cats

Idiopathic hypercalcemia is a feline-specific condition with no equivalent in dogs. Ionized calcium is persistently elevated, but PTH is suppressed (low) — the parathyroid glands are responding correctly by shutting down PTH production, which is actually what should happen when calcium is high. The problem is that the source of calcium elevation is upstream and unknown.

• Middle-aged to older cats most affected; no strong breed predisposition
• Associated with calcium oxalate urinary stones and chronic constipation
• Theories: excess intestinal calcium absorption, altered bone calcium turnover, dietary triggers
• PTHrP is negative (no cancer signal)

Treatment options: dietary change (high-fiber, low-oxalate, or hydrolyzed protein diet), prednisolone (reduces intestinal calcium absorption), and alendronate (bisphosphonate — reduces bone resorption) for refractory cases.

See the full hyperparathyroidism in cats post for a detailed workup and treatment guide for IHC.

2. Primary Hyperparathyroidism (Parathyroid Adenoma)

A benign tumor of the parathyroid gland secretes PTH autonomously — ignoring normal feedback regulation. PTH is high (or inappropriately normal despite high calcium), ionized calcium is high, and phosphorus is low (PTH tells kidneys to waste phosphorus). The parathyroid adenoma is usually found on ultrasound as a small nodule near the thyroid.

Surgery (parathyroidectomy) is curative in most cats, but post-operative hypocalcemia is the main complication — the remaining parathyroid glands have been suppressed for months and take time to recover. Close post-surgical monitoring and calcium supplementation are required.

3. Cancer (Humoral Hypercalcemia of Malignancy)

Tumors — most commonly lymphoma in cats — secrete PTH-related protein (PTHrP), which mimics PTH and raises blood calcium. Unlike true PTH, PTHrP is not detected by standard PTH assays and requires separate testing.

Pattern: ionized calcium elevated, PTH suppressed (low), PTHrP elevated. Chest x-rays and abdominal ultrasound are key — mediastinal masses (cranial chest, pushing the trachea upward on x-ray) are classic for mediastinal lymphoma.

4. Vitamin D Toxicity

Excess vitamin D causes unregulated intestinal calcium absorption and bone resorption, driving calcium to dangerous levels. Sources in cats include:

• Vitamin D supplements: Accidental overdose or supplemented pet foods with formulation errors
• Cholecalciferol-based rodenticides: Exposure through eating a poisoned rodent (relay toxicity) or direct ingestion
• Calcipotriene (Dovonex): A human psoriasis cream containing a vitamin D analogue — cats that groom owners who applied this cream can ingest a toxic dose
• Certain plants: Jasmine (day-blooming), some Solanum species

5. Granulomatous Disease

Inflammatory granulomas (clusters of macrophages) can produce calcitriol (active vitamin D) independently of kidney regulation — raising calcium from within the granuloma. In cats this occurs most commonly with fungal disease (histoplasmosis, cryptococcosis) and occasionally with FIP. PTH is suppressed; 1,25-dihydroxyvitamin D (calcitriol) is elevated if measured. Treating the underlying infection resolves the hypercalcemia.

6. Renal Secondary Hyperparathyroidism (CKD)

In CKD, impaired phosphorus excretion and reduced calcitriol production drive PTH upward. Calcium is usually normal or low-normal in renal secondary HPT — the elevated PTH is trying to pull calcium up from bone but cannot compensate fully. Occasionally, total calcium appears slightly elevated in CKD cats. The pattern (high PTH, high phosphorus, high BUN/creatinine/SDMA, and normal or mildly elevated calcium) points to CKD rather than primary hyperparathyroidism.

1. Hypoparathyroidism — Most Common Cause

Without PTH, the kidneys cannot retain calcium and bones do not release it — blood calcium falls. In cats, the most common cause is accidental damage or removal of the parathyroid glands during thyroidectomy for hyperthyroidism. The parathyroid glands sit embedded in or adjacent to the thyroid and can be inadvertently removed or devascularized during surgery.

• Signs appear 1–5 days after thyroidectomy as calcium gradually falls
• Post-surgical monitoring of calcium is standard practice after thyroidectomy
• Treatment: IV calcium gluconate acutely; calcitriol (active vitamin D) + oral calcium long-term
• Some cats recover parathyroid function over weeks to months; others require lifelong supplementation

Primary immune-mediated hypoparathyroidism (without surgery) is rare in cats.

2. Eclampsia (Puerperal Tetany) in Nursing Queens

A nursing queen with a large litter loses calcium rapidly into her milk — faster than she can absorb it from her diet. Blood calcium drops suddenly, causing the muscle signs of hypocalcemia. Eclampsia typically occurs in the first 1–3 weeks of nursing.

3. Phosphate Enema Toxicity (Fleet Enemas)

Fleet enemas and other sodium phosphate enemas are extremely dangerous in cats. Cats rapidly absorb the phosphate through the colonic mucosa, causing a sudden massive spike in blood phosphorus. High phosphorus binds calcium, driving it precipitously low. The combination of hyperphosphatemia, hypocalcemia, and the sodium load can be fatal within hours.

4. Other Causes

• Ethylene glycol (antifreeze) toxicity: Causes calcium oxalate crystal formation, trapping calcium — while also causing severe acute kidney failure
• Acute pancreatitis: Calcium is consumed by saponification of fat (calcium soap) in areas of pancreatic fat necrosis
• Hypoalbuminemia (pseudohypocalcemia): Low albumin causes total calcium to appear low — but ionized calcium is normal, and the cat has no hypocalcemic symptoms. Confirm with ionized calcium before treating
• Hypomagnesemia: Magnesium is needed for PTH secretion and response; severe magnesium deficiency can cause hypocalcemia that does not respond to calcium supplementation until magnesium is corrected