Cortisol is a stress hormone released by the adrenal gland. Stress can be mentally or physically related. Normal release of cortisol fluctuates throughout the day. Excess cortisol production may be due to Cushing syndrome. Normal to increased levels in the morning that do not reduce by evening are often a result of Cushing syndrome. If low levels of cortisol are detected it may be due to adrenal insufficiency or Addison disease.