A 32-year-old woman presents with polyuria, fatigue, and hypercalcemia (serum Ca 11.2 mg/dL). PTH is elevated at 115 pg/mL (normal 15–65). 24-hour urine calcium is 50 mg/24 hr (low). DEXA scan shows normal bone density. What is the MOST likely diagnosis?

• A Primary hyperparathyroidism with hypercalciuria
• B Tertiary hyperparathyroidism
• C MEN-1 associated hyperparathyroidism
• D Familial hypocalciuric hypercalcemia (FHH) ✓

Explanation

Familial hypocalciuric hypercalcemia (FHH) is caused by loss-of-function mutations in the calcium-sensing receptor (CaSR) gene, leading to a higher set-point for calcium. It presents with mild hypercalcemia, elevated or inappropriately normal PTH, and characteristically low urinary calcium excretion. The calcium-to-creatinine clearance ratio (CCCR) < 0.01 confirms FHH; parathyroidectomy is NOT indicated as it will not cure the hypercalcemia. Primary hyperparathyroidism typically has elevated urine calcium, and patients with MEN-1 require additional syndromic features.

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

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Written and medically reviewed by the StethoPrep medical team.