The most common cause of hypocalcemia is renal failure, in which decreased production of active vitamin D3 and hyperphosphatemia both play a role. Some cases of primary hypoparathyroidism are due to mutation of calcium-sensing protein in which inappropriate suppression of PTH release leads to hypocalcemia.

A. Symptoms and Signs: Hypocalcemia increases excitation of nerve and muscle cells, primarily affecting the neuromuscular and cardiovascular systems. Extensive spasm of skeletal muscle causes cramps and tetany. Laryngospasm with stridor can obstruct the airway, causing fatal asphyxia. Convulsions can occur as well as paresthesias of lips and extremities and abdominal pain. Chvostek's sign (contraction of the facial muscle in response to tapping the facial nerve against the bone just anterior to the ear) and Trousseau's sign (carpal spasm occurring after occlusion of the brachial artery with a blood pressure cuff for 3 minutes) are usually readily elicited. Prolongation of the QT interval predisposes to the development of ventricular arrhythmias; heart block and ventricular fibrillation are also encountered. In chronic hypoparathyroidism, cataracts and calcification of basal ganglia of the brain may appear.

B. Laboratory Findings: Serum Ca2+ is low (< 9 mg/dL). The depressed level of serum Ca2+ must be correlated with the simultaneous concentration of serum albumin: When albumin is low, serum Ca2+ concentration is also depressed in a ratio of 0.8–1 mg of Ca2+ to 1 g of albumin. Serum phosphate is usually elevated. Serum Mg2+ is commonly low, and hypomagnesemia reduces both parathyroid hormone release and tissue responsiveness to parathyroid hormone, causing hypocalcemia. In respiratory alkalosis, total serum calcium is normal but ionized calcium is low, which can be measured by use of a Ca2+-sensitive electrode. The ECG shows a prolonged QT interval.

A. Severe, Symptomatic Hypocalcemia: In the presence of tetany, arrhythmias, or seizures, calcium gluconate 10% (10–20 mL) administered intravenously over 10–15 minutes is indicated. Because of the short duration of action, calcium infusion is usually required. Ten to 15 milligrams of calcium per kilogram body weight, or six to eight 10-mL vials of 10% calcium gluconate (558–744 mg of calcium), is added to 1 L of D5W and infused over 4–6 hours. By monitoring the serum calcium level frequently (every 4–6 hours), the infusion rate is adjusted to maintain the serum calcium level at 7–8.5 mg/dL.

B. Asymptomatic Hypocalcemia: Oral calcium and vitamin D preparations (Table 26–10) are used. Calcium carbonate is well tolerated and less expensive than many other calcium tablets. The low serum Ca2+ associated with low serum albumin concentration does not require replacement therapy. If serum Mg2+ is low, therapy must include replacement of magnesium, which by itself usually will correct hypocalcemia.