Cortisol is the primary stress hormone produced by the adrenal glands that plays a crucial role in regulating metabolism, immune function, blood pressure, and the body's response to stress. This test measures the level of cortisol in the blood to evaluate adrenal gland function and diagnose disorders of the hypothalamic-pituitary-adrenal axis. Cortisol levels follow a daily rhythm (circadian pattern), with highest levels in the early morning and lowest levels in the evening. Abnormal cortisol levels can indicate conditions such as Cushing's syndrome (excess cortisol), Addison's disease (insufficient cortisol), or pituitary gland disorders. This test is essential for evaluating patients with unexplained weight changes, fatigue, muscle weakness, high blood pressure, or other symptoms that may suggest adrenal dysfunction.