A 30-year-old man has Cushing's syndrome. Morning serum cortisol is 28 µg/dL. Low-dose dexamethasone suppression test (1 mg overnight) shows cortisol 22 µg/dL. High-dose DST (8 mg overnight) shows cortisol 15 µg/dL (46% suppression from baseline). Plasma ACTH is 64 pg/mL. The most likely source is:
- A Adrenal cortisol-secreting adenoma
- B Ectopic ACTH syndrome from bronchial carcinoid
- C Primary pigmented nodular adrenocortical disease
- D Pituitary corticotroph adenoma (Cushing's disease) ✓
Explanation
Partial suppression on high-dose DST (>50% is the traditional threshold, though > 50% of baseline) combined with detectable-elevated ACTH points to a pituitary adenoma (Cushing's disease). Adrenal adenomas are ACTH-independent (ACTH suppressed) and do not suppress on any dose. Ectopic ACTH (from small-cell lung cancer or atypical carcinoids) generally does not suppress even on high-dose DST and usually has markedly elevated ACTH. Partial suppression (~46%) in this context with mildly elevated ACTH is characteristic of Cushing's disease; further confirmation requires bilateral inferior petrosal sinus sampling.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.