A 65-year-old male smoker presents with serum sodium of 118 mEq/L, concentrated urine, and no evidence of volume depletion. Imaging reveals a central lung mass. Which paraneoplastic syndrome is most likely responsible?

• A Eaton-Lambert syndrome
• B Syndrome of inappropriate antidiuretic hormone ✓
• C Hypercalcemia of malignancy
• D Cushing syndrome

Explanation

SIADH is the most common cause of euvolemic hyponatremia in cancer patients and is classically associated with small cell lung cancer. The tumor ectopically produces ADH, leading to water retention and dilutional hyponatremia. Eaton-Lambert causes proximal muscle weakness. Hypercalcemia is seen with squamous cell lung cancer via PTHrP. Cushing syndrome results from ectopic ACTH production, also in small cell lung cancer, but presents with hypokalemia and metabolic alkalosis.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.