A 35-year-old woman is found to have a serum calcium of 11.2 mg/dL on a routine check. PTH is 82 pg/mL (elevated). 24-hour urinary calcium is 75 mg/day (low). Urine calcium-to-creatinine clearance ratio is 0.005. What is the most likely diagnosis?

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

Explanation

A urine calcium-to-creatinine clearance ratio (UCCR = [urine Ca × serum Cr] / [serum Ca × urine Cr]) below 0.01, combined with hypercalcemia and appropriately elevated or high-normal PTH, is the hallmark of FHH — an autosomal dominant condition due to inactivating mutations in the calcium-sensing receptor (CaSR) gene. Unlike primary hyperparathyroidism, FHH patients have reduced renal calcium excretion. Parathyroidectomy is contraindicated and ineffective in FHH. Primary hyperparathyroidism would show elevated urinary calcium.

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

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Written and medically reviewed by the StethoPrep medical team.