A 62-year-old man admitted with community-acquired pneumonia has serum sodium 118 mEq/L, plasma osmolality 252 mOsm/kg, urine osmolality 420 mOsm/kg, urine sodium 65 mEq/L, and normal renal, adrenal and thyroid function. He is euvolaemic. What does this profile indicate?
- A Syndrome of inappropriate antidiuretic hormone secretion with inappropriately concentrated urine ✓
- B Hypovolaemic hyponatraemia with sodium-retaining kidneys
- C Primary polydipsia with appropriately dilute urine
- D Cerebral salt wasting requiring aggressive isotonic volume repletion
Correct answer: A. Syndrome of inappropriate antidiuretic hormone secretion with inappropriately concentrated urine
Explanation
With hypo-osmolality, a normally functioning hypothalamic-pituitary axis should suppress ADH and produce maximally dilute urine below 100 mOsm/kg. Here the urine is concentrated at 420 mOsm/kg with high urine sodium in a euvolaemic patient whose other endocrine axes are intact, fulfilling diagnostic criteria for SIADH, commonly caused in this setting by pneumonia. Primary polydipsia gives dilute urine, and hypovolaemic states produce clinical volume depletion, which this patient lacks.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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