A 58-year-old man with small cell lung cancer has serum sodium 116 mEq/L, serum osmolality 245 mOsm/kg, urine osmolality 480 mOsm/kg, urine sodium 80 mEq/L, and normal renal, adrenal, and thyroid function. He is euvolaemic. The most appropriate initial management is:

• A Isotonic saline infusion
• B 3% hypertonic saline bolus
• C Demeclocycline
• D Fluid restriction ✓

Explanation

These findings establish SIADH: hyponatraemia with inappropriately concentrated urine, urinary sodium loss, euvolaemia, and excluded endocrine causes. First-line therapy in asymptomatic or mildly symptomatic chronic SIADH is free water restriction, typically under 1 L/day. Isotonic saline worsens it because the retained water dilutes the sodium further. Hypertonic saline is reserved for seizures or severe symptoms, and demeclocycline or vaptans are second-line options.

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

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Written and medically reviewed by the StethoPrep medical team.