A 55-year-old smoker with small-cell lung carcinoma develops hyponatremia (serum Na 118 mEq/L) with inappropriately elevated urine osmolality. The mediator responsible for this paraneoplastic syndrome is:

• A Ectopic ACTH secretion
• B Parathyroid hormone-related protein (PTHrP)
• C Ectopic erythropoietin secretion
• D Ectopic ADH (vasopressin) secretion ✓

Explanation

SIADH (syndrome of inappropriate antidiuretic hormone secretion) caused by ectopic ADH production by small-cell lung carcinoma leads to dilutional hyponatremia with high urine osmolality relative to serum. Ectopic ACTH from SCLC causes Cushing syndrome, not hyponatremia. PTHrP causes hypercalcemia (commonest cause of paraneoplastic hypercalcemia). Ectopic EPO causes erythrocytosis.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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