A 55-year-old man with non-small cell lung cancer develops hyponatremia (serum Na⁺ 118 mEq/L) with urinary sodium >20 mEq/L and urine osmolality >100 mOsm/kg in the absence of adrenal or thyroid dysfunction. Immunohistochemistry of the tumor shows expression of which hormone most directly responsible for this paraneoplastic syndrome?

• A ACTH
• B PTHrP
• C Erythropoietin
• D Arginine vasopressin (AVP/ADH) ✓

Explanation

Ectopic AVP (ADH) secretion from SCLC (and occasionally NSCLC) causes SIADH — euvolemic hyponatremia with inappropriately concentrated urine and natriuresis. PTHrP causes hypercalcemia; ectopic ACTH causes Cushing syndrome; erythropoietin causes polycythemia. The lab pattern here (low serum Na, high urine Na, high urine osmolality, euvolemia) is classic SIADH.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.