A female patient has a pituitary adenoma secreting ACTH (Cushing's disease). Which finding on dynamic testing would MOST reliably distinguish this from ectopic ACTH secretion?
- A Low-dose dexamethasone (1 mg overnight) suppresses cortisol in Cushing's disease but not ectopic ACTH
- B High-dose dexamethasone (8 mg) suppresses cortisol >50% in Cushing's disease but NOT in ectopic ACTH secretion ✓
- C CRH stimulation test shows no ACTH rise in Cushing's disease but a paradoxical rise in ectopic ACTH
- D Insulin tolerance test (ITT) shows no cortisol response in both Cushing's disease and ectopic ACTH secretion
Explanation
The high-dose dexamethasone suppression test (HDDST, 8 mg overnight or 2-day test) distinguishes pituitary-dependent Cushing's disease from ectopic ACTH syndrome. Pituitary corticotroph adenomas retain partial glucocorticoid feedback sensitivity — high-dose dexamethasone suppresses ACTH and cortisol by >50% in approximately 80-90% of Cushing's disease cases. Ectopic ACTH-secreting tumours (lung carcinoid, SCLC) are autonomous and do not suppress with dexamethasone. Low-dose DST fails to suppress in BOTH conditions (they share resistance to low-dose suppression). CRH stimulation shows an ACTH rise in Cushing's disease but no response in ectopic ACTH.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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