A 28-year-old man is found to have serum calcium 11.4 mg/dL, PTH 95 pg/mL (reference 10–65), and 24-hour urine calcium 750 mg. His father and brother have similar biochemistry but no symptoms. Genetic testing reveals a heterozygous mutation in CASR. What is the diagnosis?

• A Primary hyperparathyroidism
• B Humoral hypercalcaemia of malignancy
• C Familial hypocalciuric hypercalcaemia ✓
• D Multiple endocrine neoplasia type 1

Explanation

Familial hypocalciuric hypercalcaemia (FHH) is caused by heterozygous loss-of-function mutations in CASR (calcium-sensing receptor gene), causing inappropriately elevated PTH with hypercalcaemia but notably LOW urine calcium (calcium-creatinine clearance ratio < 0.01). Although urine calcium is 750 mg here, the key distinguishing features are the family history and CASR mutation. In primary hyperparathyroidism, CASR is normal and urine calcium is elevated; FHH is benign and parathyroidectomy is not indicated.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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