A 29-year-old woman is found to have hypercalcaemia (serum Ca 11.2 mg/dL) on routine labs. PTH is 98 pg/mL (normal 10–65). Urinary calcium:creatinine clearance ratio (UCCR) is 0.005 (normal >0.01). Her mother and aunt both have asymptomatic hypercalcaemia. What is the most likely diagnosis?
- A Primary hyperparathyroidism
- B Tertiary hyperparathyroidism
- C Familial hypocalciuric hypercalcaemia (FHH) ✓
- D MEN-1 associated parathyroid adenoma
Correct answer: C. Familial hypocalciuric hypercalcaemia (FHH)
Explanation
Familial hypocalciuric hypercalcaemia (FHH) is caused by loss-of-function mutations in the calcium-sensing receptor (CaSR), resulting in a reset upward of the calcium set-point. UCCR <0.01 is the key discriminator; primary hyperparathyroidism typically shows UCCR >0.02. PTH is mildly elevated or inappropriately normal in FHH. It is an autosomal dominant, benign condition requiring no parathyroid surgery. Parathyroidectomy in FHH will not cure hypercalcaemia.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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