A patient with Cushing's disease undergoes pituitary surgery and achieves remission. Post-operatively, morning serum cortisol is 1.2 µg/dL. Which finding in this patient best indicates successful surgical cure?

• A Normal 24-hour urine free cortisol within one week
• B Resolution of hypertension on the first post-operative day
• C Return of ACTH to the normal range within 48 hours
• D Post-operative serum cortisol < 2 µg/dL (adrenal insufficiency) indicating tumour removal ✓

Explanation

Post-operative serum cortisol < 2 µg/dL within 1–7 days of pituitary surgery for Cushing's disease is the most reliable biochemical marker of surgical cure, reflecting suppression of the normal corticotroph cells by the previously elevated cortisol. This requires cortisol replacement. Early normalisation of UFC or ACTH is less reliable; blood pressure resolution can lag weeks.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.