SIADH vs DI

Stop and Think

Your patient is post-op day 1 from a transsphenoidal hypophysectomy. Over the last 3 hours, urine output has been 300 mL/hr and climbing. Urine specific gravity is 1.002. Serum sodium has risen from 140 to 147 mEq/L. The patient reports intense thirst.

Two mirror-image conditions can follow pituitary surgery. Which one is this?

Your Turn

Given this trend, which action is the priority?

• A Restrict fluids to 800–1000 mL/day. That's the SIADH protocol. This patient is losing fluid, not retaining it.
• B Notify the provider, replace fluid losses, and prepare for possible desmopressin. Correct. High output, dilute urine, and rising sodium point to diabetes insipidus.
• C Give furosemide to help the kidneys clear excess fluid. There is no excess fluid here. A diuretic would worsen an already dehydrating patient.
• D Document the output as expected post-op diuresis and reassess in the morning. This output, specific gravity, and rising sodium are not routine. Waiting risks rapid dehydration.

CJMM Debrief

Why B

Pituitary surgery sits right next to the posterior pituitary, so transient diabetes insipidus after this procedure is common enough to anticipate. Without enough ADH, the kidneys stop concentrating urine, water pours out, and the blood concentrates behind it. That is the opposite failure from SIADH, where too much ADH holds water in.

• Recognize cues: Urine output 300 mL/hr, specific gravity 1.002 (dilute), sodium rising to 147, intense thirst.
• Analyze cues: High output with dilute urine and rising sodium is a water-loss pattern, not a water-retention pattern.
• Prioritize hypothesis: Diabetes insipidus, likely central, given the recent pituitary surgery.
• Take action: Replace losses, notify the provider, and anticipate DDAVP; do not restrict fluids.

keep asking why