Which statement best differentiates primary hyperparathyroidism from familial hypocalciuric hypercalcemia in terms of urinary calcium excretion?

Which statement best differentiates primary hyperparathyroidism from familial hypocalciuric hypercalcemia in terms of urinary calcium excretion?

• A. FHH typically has hypercalciuria; PHPT has hypocalciuria.
• B. Urinary calcium excretion is not helpful in differentiating these conditions.
• C. PHPT typically has hypercalciuria; FHH has hypocalciuria.
• D. Both conditions always present with normal urinary calcium excretion.

Answer: (C)

Urinary calcium excretion is the key clue that helps tell these two conditions apart. In primary hyperparathyroidism, excess PTH raises serum calcium and the kidney ends up excreting more calcium, so urine calcium is typically high (hypercalciuria). In contrast, familial hypocalciuric hypercalcemia results from a defective calcium-sensing receptor in the kidney, which makes the kidney reabsorb more calcium; this leads to low urine calcium (hypocalciuria) despite high or normal serum calcium. Because of this opposite pattern, the statement that primary hyperparathyroidism usually has hypercalciuria and familial hypocalciuric hypercalcemia usually has hypocalciuria is the best choice. The urinary calcium-to-creatinine ratio often supports this distinction, with a low ratio favoring FHH and a higher ratio favoring PHPT.

Urinary calcium excretion is the key clue that helps tell these two conditions apart. In primary hyperparathyroidism, excess PTH raises serum calcium and the kidney ends up excreting more calcium, so urine calcium is typically high (hypercalciuria). In contrast, familial hypocalciuric hypercalcemia results from a defective calcium-sensing receptor in the kidney, which makes the kidney reabsorb more calcium; this leads to low urine calcium (hypocalciuria) despite high or normal serum calcium. Because of this opposite pattern, the statement that primary hyperparathyroidism usually has hypercalciuria and familial hypocalciuric hypercalcemia usually has hypocalciuria is the best choice. The urinary calcium-to-creatinine ratio often supports this distinction, with a low ratio favoring FHH and a higher ratio favoring PHPT.