A 28-year-old woman is found to have elevated 24-hour urine free cortisol and fails to suppress on overnight low-dose dexamethasone (1 mg). High-dose dexamethasone (8 mg) suppresses cortisol by 60%. What is the most likely diagnosis?

• A Ectopic ACTH syndrome (e.g., small cell lung carcinoma)
• B Primary adrenal adenoma (ACTH-independent Cushing's syndrome)
• C Pseudo-Cushing's syndrome from chronic alcoholism
• D Pituitary ACTH-secreting adenoma (Cushing's disease) ✓

Explanation

The dexamethasone suppression test differentiates Cushing's syndrome subtypes. Pituitary adenomas (Cushing's disease) retain partial negative feedback sensitivity — they do NOT suppress on low-dose DST (1 mg or 2-day 2 mg/day), but DO suppress cortisol by ≥50% on high-dose DST (8 mg overnight or 2-day 2 mg qid). Ectopic ACTH (e.g., SCLC) shows autonomous ACTH production that fails to suppress even on high-dose DST. Adrenal adenomas are ACTH-independent — plasma ACTH is suppressed. Pseudo-Cushing's also shows partial suppression on high-dose DST but in a different clinical context.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.