A cortisol test may be used to help diagnose Cushing syndrome, a condition associated with excess cortisol, or to help diagnose adrenal insufficiency or Addison disease, conditions associated with deficient cortisol. Cortisol is a hormone that plays a role in the metabolism of proteins, lipids, and carbohydrates, among other functions. Normally, the level of cortisol in the blood rises and falls in a "diurnal variation" pattern, peaking early in the morning, then declining throughout the day and reaching its lowest level about midnight. Cortisol is produced and secreted by the adrenal glands. Production of the hormone is regulated by the hypothalamus in the brain and by the pituitary gland, a tiny organ located below the brain. When the blood cortisol level falls, the hypothalamus releases corticotropin-releasing hormone (CRH), which directs the pituitary gland to produce ACTH (adrenocorticotropic hormone). ACTH stimulates the adrenal glands to produce and release cortisol.