A 32-year-old man develops polyuria and polydipsia after transsphenoidal surgery for a pituitary adenoma. Urine output is 8 L per day. Serum sodium is 152 mEq/L, serum osmolality is 310 mOsm per kg, and urine osmolality is 100 mOsm per kg. After 6 hours of water deprivation, urine osmolality remains at 110 mOsm per kg. Following desmopressin administration, urine osmolality rises to 450 mOsm per kg. What is the diagnosis?

• A Nephrogenic diabetes insipidus
• B Osmotic diuresis
• C Primary polydipsia
• D Central diabetes insipidus ✓

Explanation

The water deprivation test shows failure to concentrate urine despite hypernatremia and elevated serum osmolality, confirming diabetes insipidus. The response to desmopressin with urine osmolality rising more than 50 percent above baseline confirms central diabetes insipidus, where the kidney can respond to exogenous ADH. In nephrogenic diabetes insipidus, there is minimal response to desmopressin. Primary polydipsia shows concentrated urine after water deprivation.

Reference: Williams Textbook of Endocrinology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.