In differentiating primary hyperparathyroidism from familial hypocalciuric hypercalcemia, what does 24-hour urinary calcium typically show?

In differentiating primary hyperparathyroidism from familial hypocalciuric hypercalcemia, what does 24-hour urinary calcium typically show?

• A. PHPT low urinary calcium; FHH high urinary calcium
• B. Both high
• C. Primary hyperparathyroidism elevated urinary calcium; familial hypocalciuric hypercalcemia has low urinary calcium excretion.
• D. Both low

Answer: (C)

The key idea is how calcium is handled by the kidneys in these two conditions. In primary hyperparathyroidism, excess PTH raises serum calcium through bone resorption and increased intestinal absorption, and the elevated calcium load leads to more being excreted in the urine. So 24-hour urinary calcium is typically elevated. In familial hypocalciuric hypercalcemia, a defective calcium-sensing receptor makes the kidneys less responsive to calcium, so the kidneys reclaim more calcium instead of wasting it. That results in low urinary calcium despite high calcium levels. So, a higher 24-hour urinary calcium points to PHPT, while a low value suggests FHH.

The key idea is how calcium is handled by the kidneys in these two conditions. In primary hyperparathyroidism, excess PTH raises serum calcium through bone resorption and increased intestinal absorption, and the elevated calcium load leads to more being excreted in the urine. So 24-hour urinary calcium is typically elevated.

In familial hypocalciuric hypercalcemia, a defective calcium-sensing receptor makes the kidneys less responsive to calcium, so the kidneys reclaim more calcium instead of wasting it. That results in low urinary calcium despite high calcium levels.

So, a higher 24-hour urinary calcium points to PHPT, while a low value suggests FHH.