Which test is completed after high cortisol levels are found in Cushing's syndrome?

In the evaluation of Cushing's syndrome, after high cortisol levels have been confirmed, the next logical step is to perform a dexamethasone suppression test. This test is crucial for distinguishing between different causes of elevated cortisol levels, particularly to determine whether the rise in cortisol is due to a pituitary adenoma (Cushing's disease), ectopic ACTH production, or adrenal tumors.

The dexamethasone suppression test works by administering dexamethasone, a synthetic glucocorticoid, which normally suppresses cortisol production in healthy individuals. If cortisol levels are not adequately suppressed after administration of dexamethasone, it suggests that there is an abnormal source of cortisol production, such as in Cushing's syndrome. The response can help clarify if the source of cortisol is pituitary (often resulting in suppression) or ectopic/adrenal (typically showing lack of suppression).

The other tests listed serve different purposes and are not immediate follow-ups after confirming high cortisol levels. For example, the ACTH stimulation test assesses adrenal gland response to ACTH, and a fast-acting insulin test and insulin suppression test are related to glucose metabolism rather than adrenal function or cortisol levels. Therefore, performing the dexamethasone suppression