Familial hypocalciuric hypercalcemia is characterized by what urinary calcium finding?

• A. Hypercalciuria
• B. Normal urinary calcium
• C. Hypercalciuria with nephrolithiasis
• D. Hypocalciuria

Answer: (D)

Familial hypocalciuric hypercalcemia arises from inactivating mutations of the calcium-sensing receptor in both the parathyroid and kidney. The defective receptor causes the kidney to reabsorb calcium more avidly, so calcium is not excreted in the urine as it normally would be despite high serum calcium. This leads to hypocalciuria—low urinary calcium—even in the setting of hypercalcemia. Clinically, you’d see a low fractional excretion of calcium, which helps differentiate this condition from primary hyperparathyroidism, where urinary calcium is typically normal or elevated.