Which test is used to differentiate causes of Cushing's syndrome?

The dexamethasone suppression test is instrumental in differentiating the causes of Cushing's syndrome. This test leverages the feedback mechanisms in the hypothalamic-pituitary-adrenal (HPA) axis. In a healthy individual, administering high doses of dexamethasone, a synthetic glucocorticoid, typically suppresses cortisol production. If cortisol levels remain elevated despite this suppression, it indicates Cushing's syndrome, possibly due to a pituitary adenoma (Cushing's disease), adrenal tumors, or ectopic ACTH production.

By interpreting the results of this test, clinicians can ascertain whether the source of the high cortisol levels is pituitary (Cushing's disease), adrenal (primary hypercortisolism), or ectopic (produced by non-pituitary tumors). This makes the dexamethasone suppression test crucial for establishing the subtype of Cushing's syndrome.

The other tests listed serve different purposes. The ACTH stimulation test primarily evaluates whether the adrenal glands are functioning properly, while the 24-hour urine cortisol test quantifies total cortisol output but does not specify the source. Cortisol level testing provides an initial measure of cortisol but does not help differentiate between various causes. Therefore, the dex