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Syndrome of inappropriate antidiuresis — RACP Adult Medicine MCQ

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HardEndocrinology and diabetesSyndrome of inappropriate antidiuresisRACP Adult Medicine

A 69-year-old smoker presents with confusion. Sodium is 118 mmol/L, serum osmolality is low, urine osmolality is 520 mOsm/kg and urine sodium is 64 mmol/L. He is clinically euvolaemic and chest CT shows a central hilar mass. What is the most likely cause?

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Correct answer: B — Ectopic antidiuretic hormone secretion from small cell lung cancer

Euvolaemic hypotonic hyponatraemia with inappropriately concentrated urine and a central lung mass suggests SIADH from small cell lung cancer. Adrenal failure usually has hypotension and hyperkalaemia. Cerebral salt wasting, osmotic diuresis and GI losses are typically hypovolaemic or hyperosmolar patterns.

Reference: Therapeutic Guidelines (eTG) Endocrinology; AMH