In the high-dose dexamethasone suppression test, which pattern best distinguishes Cushing's disease from ectopic ACTH production?

• A. Both show suppression
• B. Only Cushing's disease shows cortisol suppression
• C. Only ectopic shows cortisol suppression
• D. Neither shows cortisol suppression

Answer: (B)

The test distinguishes sources of excess ACTH by whether cortisol production can be suppressed when a high dose of dexamethasone is given. In Cushing’s disease, the pituitary tumor ACTH production retains some sensitivity to negative feedback, so a high dose of dexamethasone reduces ACTH and lowers cortisol levels. In ectopic ACTH production, tumors outside the pituitary secrete ACTH autonomously and are not suppressed by dexamethasone, so cortisol remains elevated. Thus the pattern you’d expect is cortisol suppression with high-dose dexamethasone in Cushing’s disease, and no suppression in ectopic ACTH syndrome. This is the best discriminator between the two conditions, though there are occasional exceptions.