A 30-year-old man is found to have a serum cortisol of 850 nmol/L at 8 AM. A 1 mg overnight dexamethasone suppression test shows a cortisol of 180 nmol/L. Urinary free cortisol is three times the upper limit. ACTH is suppressed at <5 pg/mL. The most likely diagnosis is:
- A Adrenal adenoma secreting cortisol ✓
- B Pituitary Cushing's disease
- C Ectopic ACTH syndrome
- D Pseudo-Cushing's syndrome due to alcoholism
Correct answer: A. Adrenal adenoma secreting cortisol
Explanation
Suppressed ACTH (<5 pg/mL) in the setting of confirmed hypercortisolism localizes the source to the adrenal gland (ACTH-independent Cushing's). Pituitary Cushing's disease and ectopic ACTH syndrome are both ACTH-dependent (ACTH elevated or normal). Pseudo-Cushing's from alcoholism may show mild elevation but ACTH is typically not suppressed to this degree, and UFC is usually only modestly elevated.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.