A 22-year-old man passes 8 litres of dilute urine daily. During a supervised water deprivation test his plasma osmolality rises to 300 mOsm/kg but urine osmolality plateaus at 110 mOsm/kg. After subcutaneous desmopressin, urine osmolality rises to 430 mOsm/kg within 2 hours. What is the diagnosis?

• A Central (hypothalamic) diabetes insipidus ✓
• B Primary polydipsia
• C Syndrome of inappropriate ADH secretion
• D Nephrogenic diabetes insipidus

Explanation

Failure to concentrate urine despite adequate plasma hyperosmolality proves deficient ADH effect, and a rise in urine osmolality of more than 50% after desmopressin confirms responsiveness of the collecting ducts, establishing central DI. In nephrogenic DI the kidney cannot respond, so urine osmolality stays flat after desmopressin, which eliminates option D. In primary polydipsia the patient concentrates urine well before desmopressin, reaching 500 to 800 mOsm/kg once plasma osmolality crosses the thirst reset point.

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

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