Hypocalcemia

Dietary causes

Carnivores that are fed meat diets high in organ meats develop calcium deficiency. The resulting increase in PTH concentration promotes calcium reabsorption and phosphorus excretion by the kidneys, and bone resorption, in a condition called nutritional hyperparathyroidism. Similarly, herbivores, particularly horses receiving high grain and low hay diets, develop bone resorption from excessive phosphorus intake and calcium lack. Serum calcium and phosphorus are usually maintained within RI, although calcium may be low normal. Animals are susceptible to pathologic bone fractures due to bone resorption. Calcium may also be decreased due to decreased intake in inappetent animals.

Postparturient

Postparturient hypocalcemia (milk fever) occurs within 72 hours of calving, usually in dairy cows. In addition to low serum calcium, phosphorus and magnesium are often decreased. Calcium concentrations <1.5 mmol/L (<6 mg/dL) result in recumbency and concentrations <0.9 mmol/L (<3.6 mg/dL) may be fatal. Primary hypocalcemia is occasionally seen in beef cattle and is not necessarily associated with calving. The condition may be precipitated by a diet deficient in calcium-rich hay, particularly alfalfa, and is often a herd problem. However, cattle that are inappetent for any reason, including renal disease, often have low serum calcium due to decreased dietary intake. Differentiation between secondary hypocalcemia due to inappetence and primary hypocalcemia leading to illness and recumbency is sometimes difficult. History, clinical signs, and more severe hypocalcemia usually help distinguish primary hypocalcemia.

Postparturient hypocalcemia may be seen in horses on pasture, usually within about 10 days of foaling. In sheep, hypocalcemia can occur in the last 4-6 weeks of pregnancy and also with any severe stress or illness resulting in inappetence. Dogs and occasionally cats may develop hypocalcemia (eclampsia) within about 2-3 weeks of parturition. These animals are often twitching, nervous, and hyperventilating at presentation. Total serum calcium is usually <1.6 mmol/L (< 6.5 mg/dL). Hypoglycemia may accompany the hypocalcemia necessitating treatment with both calcium and glucose.

Primary hypoparathyroidism

Primary hypoparathyroidism is an unusual cause of hypocalcemia. Removal of hyperplastic or adenomatous thyroid glands in hyperthyroid cats sometimes results in inadvertent parathyroid gland removal, necessitating postoperative calcium supplementation. Primary hypoparathyroidism in dogs is most commonly associated with immune-mediated destruction of the parathyroid glands. Clinical signs relate to neurologic or neuromuscular abnormalities. Seizures, muscle pain, anxiety, hyperventilation, tetany, and face-rubbing may be seen. Laboratory testing reveals hypocalcemia, normal to mildly elevated phosphorus, and low PTH.

Miscellaneous causes

Hypocalcemia may occur with acute pancreatitis as discussed in Chapter 9: Digestive System. Malabsorption is an uncommon cause of hypocalcemia. Ethylene glycol nephrotoxicity is frequently associated with hypocalcemia due to calcium oxalate crystal precipitation in renal tubules.