The ACTH stimulation test is a test of the function of the adrenal glands, usually when Addison's disease or a similar condition is suspected. The patient is given a small dose of ACTH (which is normally produced by the pituitary gland) or a similar hormone. The patient's cortisol levels are then measured at regular intervals in the following hour although to distinguish between conditions the test can last up to 48 hours. In either case, a baseline ACTH level should be established as well, although test results for this hormone tend to be inaccurate. High ACTH levels also point to a problem with the adrenals, but low levels often point to a problem with the pituitary.
The test is contraindicated if the patient is on certain types of medication which affect adrenal hormone production, or in women at the end of their menstrual cycle, which affects hormone production. In addition, the patient must avoid sodium as much as possible in the 24 hours prior to the test.
In a normal patient, cortisol levels will generally double from the baseline level over a period of one hour. In patients with adrenal deficiency, the level will not rise more than 25% from a lower baseline. Levels over double indicates the problem is a secondary hormone and not the adrenal glands.