The Cortisol, Serum (3–5 PM) test measures the level of cortisol in the blood during the afternoon. Cortisol is a hormone produced by the adrenal glands that helps regulate stress response, metabolism, blood pressure, blood sugar, and immune function.

Cortisol levels follow a daily (diurnal) rhythm—they are highest in the early morning and gradually decrease throughout the day. Measuring cortisol between 3:00 and 5:00 PM helps evaluate whether this normal pattern is occurring.