What is the main difference in PTH levels between primary hyperparathyroidism and familial hypocalciuric hypercalcemia?

What is the main difference in PTH levels between primary hyperparathyroidism and familial hypocalciuric hypercalcemia?

• A. Primary hyperparathyroidism presents with low PTH and high calcium
• B. Primary hyperparathyroidism presents with elevated PTH and hypercalcemia
• C. Familial hypocalciuric hypercalcemia presents with normal or mildly elevated PTH and hypocalciuria
• D. Familial hypocalciuric hypercalcemia presents with low PTH and hypercalciuria

Answer: (B)

In hypercalcemia, how PTH behaves helps distinguish these two scenarios. Primary hyperparathyroidism involves autonomous overproduction of PTH by the parathyroid glands, so PTH remains high even with high calcium. That elevated PTH drives bone resorption, kidney reabsorption of calcium, and active vitamin D, leading to hypercalcemia. In familial hypocalciuric hypercalcemia, a calcium-sensing receptor defect shifts the calcium set-point, so PTH is not appropriately suppressed and stays normal or only mildly elevated despite high calcium. The kidneys then reabsorb calcium more avidly, causing hypocalciuria. So the key difference is that primary hyperparathyroidism presents with elevated PTH, whereas familial hypocalciuric hypercalcemia shows normal to mildly elevated PTH with hypercalcemia.

In hypercalcemia, how PTH behaves helps distinguish these two scenarios. Primary hyperparathyroidism involves autonomous overproduction of PTH by the parathyroid glands, so PTH remains high even with high calcium. That elevated PTH drives bone resorption, kidney reabsorption of calcium, and active vitamin D, leading to hypercalcemia. In familial hypocalciuric hypercalcemia, a calcium-sensing receptor defect shifts the calcium set-point, so PTH is not appropriately suppressed and stays normal or only mildly elevated despite high calcium. The kidneys then reabsorb calcium more avidly, causing hypocalciuria. So the key difference is that primary hyperparathyroidism presents with elevated PTH, whereas familial hypocalciuric hypercalcemia shows normal to mildly elevated PTH with hypercalcemia.