A 62-year-old woman presents with fatigue and constipation. Investigations reveal: serum calcium 11.8 mg/dL, PTH 95 pg/mL (normal 10–65), phosphate 2.4 mg/dL, 24-hour urine calcium 350 mg. Which single finding best distinguishes primary hyperparathyroidism from familial hypocalciuric hypercalcaemia (FHH)?
- A Elevated PTH level
- B Serum calcium above 11 mg/dL
- C 24-hour urine calcium to creatinine clearance ratio > 0.01 ✓
- D Low serum phosphate
Correct answer: C. 24-hour urine calcium to creatinine clearance ratio > 0.01
Explanation
The calcium-to-creatinine clearance ratio (CCCR = urine Ca/Cr ÷ serum Ca/Cr) differentiates FHH from primary hyperparathyroidism: CCCR < 0.01 favours FHH (benign, no surgery needed), while > 0.01 supports PHPT. In FHH, urine calcium is inappropriately low despite hypercalcaemia due to a CaSR mutation. Elevated PTH and low phosphate can occur in both conditions.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.