In Cushing's syndrome, plasma cortisol levels are expected to do what after administering dexamethasone?

In Cushing's syndrome, plasma cortisol levels are expected to increase or remain elevated after administering dexamethasone. Dexamethasone is a synthetic glucocorticoid that normally suppresses the hypothalamic-pituitary-adrenal (HPA) axis, leading to a decrease in cortisol production when administered to individuals without this syndrome.

However, in patients with Cushing's syndrome, the body's cortisol production is often due to an adrenal tumor, pituitary adenoma, or ectopic production of adrenocorticotropic hormone (ACTH). In such cases, the feedback mechanism does not function properly because of the underlying pathology, and these patients do not respond to the negative feedback signal from dexamethasone. As a result, instead of showing a decrease in cortisol levels, administering dexamethasone either results in unchanged cortisol levels or may even lead to an increase in cortisol production due to the lack of suppression of the adrenal glands.

Understanding this concept is critical, as it helps differentiate between Cushing's syndrome and other conditions that might mimic its symptoms, based on the response to dexamethasone suppression testing.