A 34-year-old woman is found to have persistent hypercalcemia (serum Ca 11.8 mg/dL) with elevated intact PTH of 92 pg/mL (normal 15–65), low-normal phosphate, and elevated urine calcium excretion. 24-hour urine calcium is 420 mg. Which diagnosis best explains this combination?
- A Familial hypocalciuric hypercalcemia (FHH)
- B Vitamin D toxicity
- C Primary hyperparathyroidism ✓
- D Tertiary hyperparathyroidism
Correct answer: C. Primary hyperparathyroidism
Explanation
Primary hyperparathyroidism is the most common cause of asymptomatic hypercalcemia in outpatients and is characterized by high PTH, high calcium, low-normal phosphate (PTH inhibits phosphate reabsorption), and hypercalciuria. FHH features inappropriately low urine calcium (calcium-to-creatinine clearance ratio <0.01) and normal PTH. Vitamin D toxicity suppresses PTH. Tertiary hyperparathyroidism occurs in the context of longstanding CKD.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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