In Cushing's disease, what is the pattern for plasma cortisol, ACTH, and response to high-dose dexamethasone suppression test?

• A. High, High, Suppressed
• B. High, Low, Suppressed
• C. High, High, Unsuppressed
• D. High, Low, Unsuppressed

Answer: (A)

In Cushing's disease, the excess cortisol comes from an ACTH-secreting pituitary adenoma, so both cortisol and ACTH are elevated. The high-dose dexamethasone suppression test works because the pituitary tumor remains partially responsive to glucocorticoid negative feedback. Giving a high dose of dexamethasone typically suppresses ACTH secretion from the pituitary, leading to a fall in cortisol levels. This pattern—high cortisol, high ACTH, and cortisol suppression with high-dose dexamethasone—distinguishes pituitary (Cushing's disease) from ectopic ACTH sources, which do not suppress, and from adrenal causes, which usually have low ACTH due to feedback.