What is the outcome of a high-dose dexamethasone suppression test in Cushing's disease?

In Cushing's disease, which is primarily caused by a pituitary adenoma secreting adrenocorticotropic hormone (ACTH), a high-dose dexamethasone suppression test results in low cortisol levels. Dexamethasone is a synthetic glucocorticoid, and its purpose in this test is to evaluate the feedback mechanism of the hypothalamic-pituitary-adrenal (HPA) axis.

In individuals with Cushing's disease, the negative feedback mechanism is impaired due to the excessive production of ACTH from the pituitary tumor. When a high dose of dexamethasone is administered, it typically suppresses cortisol production in individuals without Cushing's disease or in cases of ectopic ACTH syndrome, but it can partially suppress cortisol production in Cushing's disease. This suppression indicates that while there is still an abnormality in cortisol production due to the pituitary adenoma, a high dose can still exert some regulatory control.

The result of low cortisol levels after high-dose dexamethasone administration is an important diagnostic marker, confirming that the hypercortisolism is indeed due to Cushing's disease rather than other causes such as adrenal tumors or ectopic ACTH production, where you would