The Serum Cortisol Test (3-5 PM) measures the level of cortisol in the blood, specifically collected in the afternoon, typically between 3:00 and 5:00 PM. Cortisol is a hormone produced by the adrenal glands, and its levels fluctuate throughout the day, following a natural rhythm known as the diurnal pattern. Cortisol levels are usually highest in the morning and lowest in the evening. Measuring cortisol levels in the afternoon helps assess adrenal function and can be used to diagnose conditions like Cushing’s syndrome (excess cortisol) or adrenal insufficiency (low cortisol). Abnormal results may indicate a disruption in the body’s normal cortisol production cycle.