Which biochemical finding BEST distinguishes PTH-independent hypercalcemia (e.g., malignancy-associated) from primary hyperparathyroidism?
- A Elevated serum phosphate with elevated calcium
- B Suppressed intact PTH (<10 pg/mL) with hypercalcemia ✓
- C Elevated alkaline phosphatase
- D Hypercalciuria on 24-hour urine
Correct answer: B. Suppressed intact PTH (<10 pg/mL) with hypercalcemia
Explanation
In primary hyperparathyroidism, PTH is elevated or inappropriately normal despite hypercalcemia. In malignancy-associated hypercalcemia (via PTHrP secretion, osteolysis, or calcitriol excess), intact PTH is suppressed below 10 pg/mL because the negative feedback from hypercalcemia overrides any PTH drive. Serum phosphate is often low in PHPT (PTH causes phosphaturia) and also in PTHrP-mediated hypercalcemia. ALP may be elevated in both. Hypercalciuria occurs in both conditions.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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