ACTH (Adrenocorticotropic hormone) is a hormone produced by the pituitary gland in the brain that stimulates the adrenal glands to produce cortisol. A blood test is used to measure ACTH levels in the body. The test involves inserting a needle into a vein in the arm and collecting a sample of blood, which is then sent to a laboratory for analysis. Abnormal ACTH levels can indicate conditions such as Addisons disease, Cushings syndrome, or pituitary gland disorders.