Hypoparathyroidism

Parathyroid hormone (PTH) normally protects the extracellular calcium concentration. It increases calcium release and reabsorption, promotes renal calcium conservation, reduces phosphate reabsorption, and supports conversion of vitamin D to its active form, 1,25-dihydroxyvitamin D. When PTH is absent or insufficient, serum calcium falls while phosphate rises. Less active vitamin D is produced, so intestinal calcium absorption also declines. The result is neuromuscular excitability: nerves and muscles fire too easily when ionized calcium is low. This explains the clinical picture. Mild hypocalcemia causes tingling around the mouth or in the fingers, muscle cramps, and fatigue. A larger or more rapid fall can produce carpopedal spasm, tetany, laryngospasm, seizures, a prolonged QT interval, and dysrhythmias. The seriousness of hypocalcemia depends on both the calcium concentration and the patient’s symptoms. A patient with gradually developing low calcium may appear relatively stable, whereas a patient whose calcium drops quickly after neck surgery can deteriorate rapidly.