In the test, dexamethasone, a steroid, is injected into the patient. After that, the patient's cortisol level is measured. Normally, dexamathasone acts on the pituitary to suppress the action of ACTH. As such, a normal patient injected with a small dose of dexamethasone will have their cortisol levels fall. If the patient has a cortisol related illness, the cortisol levels will generally not change. The patient is then given a higher dose of dexamethasone. In patient's with Cushing's this will lower cortisol levels, but if they are still unchanged, it is likely a different condition is responsible.