In familial hypocalciuric hypercalcemia, PTH level is best described as?

• A. Normal to slightly high
• B. Elevated markedly
• C. Suppressed
• D. Undetectable

Answer: (A)

In familial hypocalciuric hypercalcemia, a loss-of-function mutation in the calcium-sensing receptor makes the parathyroid glands and kidney “think” calcium is lower than it really is. Because the calcium-sensing system is reset, the normal feedback that should suppress PTH when calcium rises is blunted. As a result, PTH secretion stays in the normal range or only mildly elevated despite hypercalcemia. This contrasts with conditions like primary hyperparathyroidism, where PTH is typically clearly elevated, and with situations where PTH would be suppressed. The key takeaway is that PTH is not appropriately suppressed in FHH; it remains normal to slightly high. The accompanying hypocalciuria (low urinary calcium) is another characteristic feature that reflects the same CaSR defect in the kidney.