A routine check reveals a mild, new elevation in calcium in an older patient; which test should be ordered next to evaluate suspected primary hyperparathyroidism?

• A. 1,25-dihydroxyvitamin D
• B. Phosphate
• C. Intact parathyroid hormone
• D. Thyroid ultrasound

Answer: (C)

Measuring intact parathyroid hormone is the immediate step when hypercalcemia is suspected to be due to primary hyperparathyroidism. In PHPT, the calcium is high because PTH is inappropriately elevated or not adequately suppressed for the elevated calcium level. If the PTH level is high or inappropriately normal, PHPT is likely and you would then pursue localization studies to find the adenoma (such as a sestamibi scan or neck ultrasound). If PTH is suppressed in the setting of hypercalcemia, that points to a PTH-independent cause (for example, malignancy with PTHrP, vitamin D–mediated processes, or granulomatous disease), guiding further testing like PTHrP, 1,25-dihydroxy vitamin D, and vitamin D levels.