A 40-year-old man has polyuria of 8 L/day and serum sodium of 152 mEq/L. After water deprivation confirms complete antidiuresis failure, he receives 4 micrograms of desmopressin intravenously. Urine osmolality remains unchanged at 90 mOsm/kg over the next two hours. Which finding best supports the underlying diagnosis?
- A Normalisation of urine concentration with hydrochlorothiazide plus amiloride ✓
- B Response of polyuria to thiazide diuretic withdrawal
- C Rapid correction of hypernatraemia after free water restriction alone
- D Absence of family history excludes this condition
Correct answer: A. Normalisation of urine concentration with hydrochlorothiazide plus amiloride
Explanation
No rise in urine osmolality after desmopressin indicates nephrogenic diabetes insipidus from renal vasopressin V2 receptor insensitivity. Thiazides induce mild volume contraction that reduces free water delivery, and amiloride counters lithium induced downregulation of aquaporin channels, so this combination partially concentrates urine in nephrogenic DI. Free water restriction cannot work when the kidney cannot respond. Lithium induced nephrogenic DI may occur without any family history.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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