Which test is typically used to diagnose Cushing's syndrome?

The diagnosis of Cushing's syndrome often involves the use of the Dexamethasone suppression test, which is designed to evaluate the body's cortisol production and response to corticosteroids. In a typical scenario, a low dose of dexamethasone is administered to suppress the secretion of cortisol from the adrenal glands. In healthy individuals, this suppression occurs, leading to a decrease in plasma cortisol levels. However, in patients with Cushing's syndrome, the suppression is inadequate, indicating abnormal cortisol regulation.

This test is particularly insightful because Cushing's syndrome can be caused by various factors, including pituitary adenomas, adrenal tumors, or ectopic ACTH production. The Dexamethasone suppression test helps differentiate between these potential causes by evaluating the cortisol response to the administered dexamethasone.

The other tests listed serve different purposes; for example, the Insulin tolerance test assesses insulin sensitivity and can be used in diabetes evaluation. The ACTH stimulation test evaluates the adrenal cortex's response to ACTH but is not the primary method for diagnosing Cushing's syndrome. Lastly, thyroid function tests assess thyroid hormone levels and are unrelated to the diagnosis of adrenal disorders. Thus, the Dexamethasone suppression test stands out as the primary tool