Adrenocorticotropic hormone (ACTH) is a tropic hormone produced by the anterior pituitary. The hypothalamic-pituitary axis controls it. ACTH regulates cortisol and androgen production. Diseases associated with ACTH include Addison disease, Cushing syndrome and Cushing disease. CRH is released from the hypothalamus. CRH stimulates the anterior pituitary to release ACTH. ACTH acts on the adrenal cortex to release cortisol and androgens. The increase in cortisol provides a negative feedback system to then decrease the amount of CRH released from the hypothalamus. ACTH works on G protein-coupled receptors on extracellular membranes on zona fasciculata and zona reticularis of the adrenal cortex. cAMP is the secondary messenger system. Activation of the g-couple receptor activates adenylyl cyclase, thus increase cAMP production. ACTH plays a role in glucose metabolism and immune function. The circadian rhythm influences cortisol secretion. The highest levels of cortisol are seen in the early morning, and the lowest levels are in the evening. This concept is important for diagnostic testing