A 55-year-old man with small cell lung carcinoma has serum sodium of 118 mEq/L, serum osmolality of 252 mOsm/kg, and urine osmolality of 420 mOsm/kg. He is clinically euvolemic and takes no medications. Which set of findings would be expected in this condition?

• A Hyperkalemia with low urine osmolality
• B High serum uric acid with dilute urine despite water loading
• C Peripheral edema with urine sodium below 10 mEq/L
• D Low serum uric acid with inappropriately concentrated urine ✓

Explanation

Ectopic ADH secretion (syndrome of inappropriate antidiuretic hormone) causes retention of water without sodium, giving hypoosmolar hyponatremia with euvolemia. Urine remains inappropriately concentrated relative to the diluted plasma, and expanded volume promotes uric acid excretion so serum urate is characteristically low. Edema does not occur because sodium is excreted normally, distinguishing SIADH from hypervolemic hyponatremic states.

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

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Written and medically reviewed by the StethoPrep medical team.