A cortisol level is a blood test that measures the amount of cortisol, a steroid hormone produced by the adrenal gland. The test is done to check for increased or decreased cortisol production. Cortisol is a steroid hormone released from the adrenal gland in response to ACTH, a hormone from the pituitary gland in the brain. Cortisol affects many different body systems. It plays a role in: bone, circulatory system, immune system. metabolism of fats, carbohydrates, and protein. ervous system and stress responses.