The corticotropin-releasing hormone (CRH) test is used to distinguish Cushing's disease from ectopic ACTH syndrome. In Cushing's disease, IV CRH administration causes:

• A An exaggerated ACTH and cortisol response because the pituitary corticotroph adenoma retains CRH responsiveness ✓
• B No change in ACTH or cortisol because the pituitary is autonomously secreting
• C Paradoxical cortisol suppression via negative feedback through CRH-R2
• D Blunted ACTH response identical to ectopic ACTH syndrome

Explanation

Corticotroph adenomas causing Cushing's disease retain CRH receptors and respond with an exaggerated ACTH (≥35% rise) and cortisol (≥20% rise) surge after IV CRH, because the adenoma is still partially under hypothalamic regulation. Ectopic ACTH tumors secrete ACTH autonomously and do NOT respond to exogenous CRH. This differential response forms the basis of the CRH stimulation test. There is no inhibitory CRH-R2 mechanism in this context.

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

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