A 58-year-old woman with newly diagnosed small cell lung carcinoma develops hyponatremia with serum sodium of 122 mEq/L, urine osmolality 600 mOsm/kg, and urine sodium 45 mEq/L. Which paraneoplastic mechanism is responsible?

• A Ectopic ACTH production causing mineralocorticoid excess
• B Ectopic ADH (AVP) secretion causing SIADH ✓
• C Parathyroid hormone-related peptide (PTHrP) causing hypercalcemia and secondary hyponatremia
• D Anti-Hu antibody–mediated hypothalamic destruction

Explanation

Small cell lung carcinoma is the classic cause of SIADH via ectopic arginine vasopressin (ADH) secretion, resulting in euvolemic hyponatremia with inappropriately concentrated urine. The low urine osmolality threshold (>100 mOsm/kg relative to low serum osmolality) and high urine sodium confirm SIADH. Ectopic ACTH from SCLC causes Cushing's syndrome, not hyponatremia primarily, and PTHrP causes hypercalcemia rather than hyponatremia via direct osmotic mechanisms.

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

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