What is the purpose of the dexamethasone suppression test in Cushing's syndrome?

The dexamethasone suppression test is primarily utilized to help determine the source of excess cortisol in patients suspected of having Cushing's syndrome. In this test, a dose of dexamethasone, a synthetic glucocorticoid, is administered, and the body's response is measured, specifically the cortisol levels in the plasma.

In a healthy individual, the administration of dexamethasone inhibits the release of adrenocorticotropic hormone (ACTH), which in turn suppresses cortisol secretion from the adrenal glands. However, in cases of Cushing's syndrome, particularly when the excess cortisol is due to an adrenal cause (adrenal adenoma) or ectopic ACTH secretion, the cortisol levels often remain elevated despite dexamethasone administration.

The results of this test can indicate whether the source of Cushing’s syndrome is from a pituitary adenoma (Cushing's disease), which would typically show suppression with overnight high-dose dexamethasone, or other causes associated with adrenal or ectopic production where suppression is absent. Thus, this test is crucial for differentiating among the various etiologies of cortisol excess, ultimately guiding further investigations and management of the condition effectively.

Understanding the specific physiological responses