Three days after transsphenoidal surgery for a nonfunctioning pituitary macroadenoma, a 46-year-old man develops polyuria of 6 L/day with serum sodium 148 mEq/L. Urine osmolality is 110 mOsm/kg. After subcutaneous desmopressin, urine output falls sharply and urine osmolality rises to 520 mOsm/kg. What is the diagnosis?

• A Nephrogenic diabetes insipidus
• B Primary polydipsia
• C SIADH with secondary polyuria
• D Central diabetes insipidus ✓

Explanation

Post-transsphenoidal polyuria with dilute urine and hypernatremia indicates diabetes insipidus. A brisk fall in urine output with a rise in urine osmolality exceeding 50% after desmopressin confirms vasopressin deficiency, that is central DI. In nephrogenic DI the kidney cannot respond to desmopressin, so osmolality stays below 300 mOsm/kg. SIADH causes concentrated urine and hyponatremia, the opposite picture.

Reference: Williams Textbook of Endocrinology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.