A 44-year-old woman is found to have a serum calcium of 11.6 mg/dL on routine testing. PTH is 92 pg/mL (normal 10–65), phosphate is low-normal, and 24-hour urinary calcium is 380 mg/day. She is asymptomatic. Which condition does this biochemical profile most specifically indicate?
- A Familial hypocalciuric hypercalcaemia
- B Humoral hypercalcaemia of malignancy
- C Primary hyperparathyroidism ✓
- D Vitamin D toxicity
Correct answer: C. Primary hyperparathyroidism
Explanation
Elevated PTH with hypercalcaemia, low-normal phosphate, and urinary calcium >250 mg/day (here 380 mg/day) is consistent with primary hyperparathyroidism. Familial hypocalciuric hypercalcaemia (FHH) presents with PTH in the normal-to-mildly elevated range and characteristically LOW urinary calcium (calcium-to-creatinine clearance ratio <0.01). Humoral hypercalcaemia of malignancy is due to PTHrP, which suppresses intact PTH. Vitamin D toxicity suppresses PTH.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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