Final answer:
In a Metyrapone stimulation test, a decrease in cortisol synthesis prompts an intact HPA axis to reactively increase ACTH levels.
Step-by-step explanation:
The Metyrapone stimulation test is designed to evaluate the integrity of the hypothalamic-pituitary-adrenocortical (HPA) axis by inhibiting cortisol synthesis. When metyrapone reduces cortisol levels, the pituitary gland should respond by increasing the secretion of adrenocorticotropic hormone (ACTH). This occurs because metyrapone blocks the enzyme 11-beta-hydroxylase, which is necessary for cortisol synthesis, leading to a decrease in cortisol levels. An intact HPA axis will detect this drop in cortisol and, via a negative feedback mechanism, stimulate the release of corticotropin releasing hormone (CRH) from the hypothalamus, which in turn prompts the anterior pituitary gland to secrete more ACTH to try and increase cortisol levels again. Therefore, patients with an intact HPA axis will experience a reactive increase in ACTH following a Metyrapone test.