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 proteinslipids, 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.