A patient with SIADH has serum Na+ of 118 mEq/L, serum osmolality 248 mOsm/kg, urine osmolality 620 mOsm/kg, urine Na+ 55 mEq/L. Which feature confirms SIADH rather than hypovolaemic hyponatraemia?
- A Urine osmolality > serum osmolality
- B Low serum Na+ and low serum osmolality
- C Urine Na+ > 40 mEq/L in the context of euvolaemia (not hypovolaemia) with normal renal, adrenal, and thyroid function ✓
- D Urine specific gravity > 1.020
Correct answer: C. Urine Na+ > 40 mEq/L in the context of euvolaemia (not hypovolaemia) with normal renal, adrenal, and thyroid function
Explanation
Both SIADH and hypovolaemic hyponatraemia can have high urine osmolality and low serum osmolality. The key discriminating feature is urine sodium and volume status. In SIADH, patients are euvolaemic (normal skin turgor, no oedema, no postural drop), and urine Na+ is typically >40 mEq/L because volume is not contracted and RAAS is not activated. In true hypovolaemia, the kidney retains sodium avidly (urine Na+ <20 mEq/L) via RAAS activation, despite the stimulus for ADH release. Assessment of volume status plus urine sodium is the diagnostic cornerstone.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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