A 55-year-old man with small cell lung carcinoma has confusion. Serum sodium is 116 mEq/L, serum osmolality is 245 mOsm/kg, urine osmolality is 550 mOsm/kg, urine sodium is 85 mEq/L, and he is clinically euvolemic without edema. Which set of expected hormonal findings fits this picture?

• A Low ADH, low atrial natriuretic peptide, high renin
• B Inappropriately high ADH, low renin, low atrial natriuretic peptide, near-normal uric acid suppressed only mildly ✓
• C Normal ADH suppressibility, high renin, high aldosterone, low urine sodium
• D High ADH, high renin, high aldosterone, expanded effective arterial volume

Explanation

Euvolemic hyponatremia with concentrated urine and high urine sodium in a patient with small cell carcinoma is SIADH: ADH is secreted independently of osmotic stimuli, so urine remains concentrated despite hypo-osmolality. Volume status is normal, hence renin and aldosterone are suppressed and there is no secondary hyperaldosteronism, killing options C and D. Hypovolemic states would show low urine sodium and activated RAAS, eliminating A. Uric acid is usually low-normal because volume expansion from retained water suppresses proximal urate reabsorption.

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

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Written and medically reviewed by the StethoPrep medical team.