A 19-year-old man has polyuria of 7 L/day and polydipsia. During a supervised water deprivation test, plasma sodium rises to 150 mEq/L and urine osmolality plateaus at 180 mOsm/kg. After desmopressin administration, urine osmolality rises to 520 mOsm/kg. What is the diagnosis?

• A Nephrogenic diabetes insipidus
• B Primary polydipsia
• C Syndrome of inappropriate antidiuresis
• D Central diabetes insipidus ✓

Explanation

Inability to concentrate urine during water deprivation followed by a rise in urine osmolality of more than 50 percent after desmopressin indicates vasopressin deficiency, that is central diabetes insipidus. In nephrogenic diabetes insipidus the kidney cannot respond, so desmopressin produces no significant rise. In primary polydipsia the patient concentrates urine normally during deprivation, and SIADH causes concentrated urine with hyponatraemia rather than hypernatraemia.

Reference: Williams Textbook of Endocrinology, 14th ed.

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