In evaluating hypercalcemia, which PTH result most strongly supports a PTH-dependent etiology?

• A. Elevated or inappropriately normal PTH.
• B. Suppressed PTH.
• C. Undetectable PTH.
• D. PTH level within the reference range.

Answer: (A)

In hypercalcemia, the body's normal response is to suppress PTH. A PTH level that is elevated or inappropriately normal (not suppressed) indicates the parathyroid glands are driving the high calcium, pointing to a PTH-dependent cause such as primary hyperparathyroidism or familial hypocalciuric hypercalcemia. If PTH is suppressed or undetectable, the hypercalcemia is PTH-independent, seen with mechanisms like PTHrP secretion by malignancies or vitamin D–mediated causes. The key idea is whether PTH remains inappropriately high or normal despite high calcium, which supports a PTH-dependent etiology.