A 55-year-old man with small cell lung carcinoma presents with confusion, nausea, and lethargy. Serum sodium is 118 mmol/L, serum osmolality 252 mOsm/kg, urine osmolality 480 mOsm/kg, urine sodium 60 mmol/L, and euvolaemia is clinically evident. The tumour cells are producing a peptide indistinguishable from which hormone?

• A ANP (atrial natriuretic peptide)
• B Aldosterone
• C ADH (vasopressin) ✓
• D PTH-related peptide

Explanation

Ectopic ADH production causes the syndrome of inappropriate antidiuresis: hypo-osmolality with inappropriately concentrated urine, euvolaemic hyponatraemia, and urinary sodium wasting. Small cell lung carcinoma is the classic source because its neuroendocrine origin allows aberrant vasopressin gene expression. ANP would produce hypovolaemia with natriuresis rather than water retention, aldosterone excess produces hypernatraemia, and PTHrP drives hypercalcaemia, all incompatible with these findings.

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

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