A patient presents to the ER after pituitary surgery with polyuria and polydipsia; what is the diagnosis?

• A. Diabetes insipidus
• B. SIADH
• C. Hyperglycemia
• D. Dehydration

Answer: (A)

Postoperative disruption of the posterior pituitary’s ability to release antidiuretic hormone leads to central diabetes insipidus. Without ADH, the kidneys can’t concentrate urine, so you get large volumes of very dilute urine and intense thirst. This fits the scenario of polyuria and polydipsia after pituitary surgery, and the patient is at risk for dehydration and rising sodium if fluids aren’t matched to losses.

SIADH would present with water retention, hyponatremia, and concentrated urine rather than polyuria. Hyperglycemia can cause polyuria via osmotic diuresis, but it’s not the typical consequence of pituitary surgery in this context. Dehydration is a result that can accompany DI, not the underlying diagnosis. If needed, a desmopressin challenge can help distinguish central DI (urine concentration improves) from other causes.