A 68-year-old man with bronchogenic carcinoma has Na+ 118 mEq/L, plasma osmolality 254 mOsm/kg, urine osmolality 520 mOsm/kg, urine Na+ 60 mEq/L, and clinically normal volume status with no oedema. The biochemical diagnosis is:

• A Syndrome of inappropriate ADH secretion ✓
• B Primary polydipsia
• C Cerebral salt wasting
• D Mineralocorticoid deficiency

Explanation

Hypotonic hyponatraemia with inappropriately concentrated urine (above roughly 100 mOsm/kg), urinary Na+ above 20 mEq/L, and euvolaemia defines SIADH. Primary polydipsia produces maximally dilute urine below 100 mOsm/kg because ADH is fully suppressed. Cerebral salt wasting shows identical laboratory values but presents with clinical hypovolaemia, which is absent here. Mineralocorticoid deficiency gives high renin, hyperkalaemia and volume depletion rather than euvolaemia.

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

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