Cortisol is a steroid hormone produced by adrenal gland in your body. It plays an important role in glucose metabolism and your body’s response to stress. Excess cortisol production by the gland leads to Cushing syndrome & low cortisol production by the gland leads to Addison’s disease. Cushing syndrome is usually due to tumors or enlargement of adrenal gland. Cortisol production is regulated by another hormone in your brain. Under this influence, cortisol is secreted in 2 peaks at 8-10 am & 4-6 pm. This test measures the morning peak.