In differentiating the causes of Cushing's, what does a high-dose dexamethasone suppression test help determine?

The high-dose dexamethasone suppression test is primarily utilized to differentiate between various causes of Cushing's syndrome, particularly distinguishing between a pituitary adenoma (Cushing's disease) and ectopic ACTH secretion (such as from a tumor). In patients with Cushing's disease, which is often due to a pituitary adenoma producing adrenocorticotropic hormone (ACTH), the high-dose dexamethasone will usually suppress cortisol production due to its effects on the pituitary gland. In contrast, ectopic ACTH production, which is independent of the pituitary control, will not be suppressed by high doses of dexamethasone.

When this test is positive for suppression, it strongly indicates a pituitary source of ACTH secretion, helping to confirm or rule out a pituitary adenoma. Thus, the high-dose dexamethasone suppression test is a critical step in assessing the origin of excess cortisol production in suspected cases of Cushing's syndrome. This differentiation is essential for guiding the appropriate management and treatment of the underlying cause.