The ACTH hormone blood test measures the levels of adrenocorticotropic hormone (ACTH) in the bloodstream. ACTH is a hormone produced by the pituitary gland that plays a crucial role in regulating the production of cortisol by the adrenal glands. This test is commonly used to evaluate the functioning of the hypothalamic-pituitary-adrenal (HPA) axis, which is responsible for regulating cortisol production.