ACTH (adrenocorticotropic hormone) is a hormone produced by the pituitary gland, located below the brain. ACTH activates glands on the kidneys (adrenal glands) to make cortisol, a substance that helps the body deal with stress.
When cortisol levels in the blood rise, the ACTH levels in a healthy person normally fall in response. When cortisol levels decrease, ACTH levels normally rise.
WebMD Medical Reference from Healthwise