A 42-year-old woman with primary hyperparathyroidism has serum calcium 11.2 mg/dL, PTH 78 pg/mL, 24-hour urine calcium 85 mg/day (low), and calcium-to-creatinine clearance ratio of 0.008. What is the most likely diagnosis?
- A Familial hypocalciuric hypercalcemia ✓
- B Secondary hyperparathyroidism due to vitamin D deficiency
- C Primary hyperparathyroidism due to single adenoma
- D Tertiary hyperparathyroidism
Correct answer: A. Familial hypocalciuric hypercalcemia
Explanation
Familial hypocalciuric hypercalcemia (FHH) is characterized by mild hypercalcemia with inappropriately normal or mildly elevated PTH, low urinary calcium excretion (24-hour urine calcium <100 mg/day), and low calcium-to-creatinine clearance ratio (<0.01). It results from an inactivating mutation in the calcium-sensing receptor gene. Parathyroidectomy is not indicated and would be ineffective, distinguishing it from primary HPT.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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