Synacthen is a synthetic form of adrenocorticotropic hormone (ACTH), which is used in diagnostic testing and for therapeutic purposes. ACTH is a hormone produced by the pituitary gland that stimulates the adrenal glands to release cortisol, a hormone essential for stress response, metabolism, and immune system regulation. Synacthen, being a synthetic analogue, mimics the action of natural ACTH, making it useful in testing the adrenal glands' function by assessing their ability to secrete cortisol in response to the stimulation. Additionally, it is used in the treatment of certain conditions such as adrenal insufficiency and congenital adrenal hyperplasia, where its role is to either supplement or stimulate the natural production of cortisol. Its application, however, is carefully monitored due to the potential for side effects and the need for precise dosing to avoid complications.