A 55-year-old man with a non-functioning pituitary adenoma undergoes transsphenoidal surgery. On postoperative day 3, he develops polyuria (8 L/day), polydipsia, and serum sodium of 148 mEq/L. What is the most likely diagnosis?

Explanation

## Central Diabetes Insipidus Post-Pituitary Surgery **Clinical Context:** Postoperative polyuria, polydipsia, and hypernatremia (148 mEq/L) following transsphenoidal surgery. **Pathophysiology:** - Pituitary surgery damages the posterior pituitary or pituitary stalk - Loss of ADH (vasopressin) secretion - Results in inability to concentrate urine - Leads to polyuria and secondary polydipsia - Hypernatremia develops if free water intake is inadequate **Key Point:** Central DI is a common complication of pituitary surgery: - Incidence: 10–15% of transsphenoidal procedures - Usually transient (resolves in days to weeks) - Can be permanent if pituitary stalk is severed **Diagnostic Clues:** - Polyuria with dilute urine (low osmolality) - Hypernatremia (not hyponatremia) - Responds to desmopressin (synthetic ADH) **Clinical Pearl:** The **triphasic response** to pituitary surgery: initial DI → transient SIADH → recovery of ADH. This patient is in phase 1.