A 28-year-old woman develops hypertension, central obesity, purple striae, and plethora. Her midnight serum cortisol is 210 nmol/L (normal < 50) and 24-hour urinary free cortisol is elevated on two separate collections. Low-dose DST shows non-suppression. High-dose DST (8 mg dexamethasone) shows cortisol suppression to 40% of baseline. The most likely diagnosis is:

• A Adrenal cortisol-secreting adenoma
• B Ectopic ACTH syndrome from small-cell lung cancer
• C Pseudo-Cushing's syndrome from alcoholism
• D Pituitary-dependent Cushing's disease (ACTH-secreting pituitary adenoma) ✓

Explanation

Cortisol suppression by >50% on high-dose DST (8 mg overnight) is characteristic of pituitary-dependent Cushing's disease, where the adenoma retains partial glucocorticoid feedback sensitivity. Adrenal adenomas show ACTH-independent hypercortisolism and do not suppress with high-dose DST. Ectopic ACTH sources are usually resistant to even high-dose suppression. Pseudo-Cushing's is excluded by the abnormal midnight cortisol and urinary findings.

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

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Written and medically reviewed by the StethoPrep medical team.