Cortisol (AM) - Blood

Cortisol (AM) is a morning blood test that measures the level of the stress hormone cortisol in your bloodstream at the time of day when it should naturally be at its highest. This early-morning peak, known as the cortisol awakening response, is your body's way of marshalling the energy and alertness you need to face the day ahead. Because blood cortisol captures a precise snapshot of your hypothalamic-pituitary-adrenal (HPA) axis activity, it is one of the most widely used clinical tools for assessing adrenal function. A morning level within the reference range suggests that the communication pathway between your brain and adrenal glands is working as expected.

The HPA axis orchestrates your cortisol production through a cascade of chemical signals. While you sleep, cortisol levels are at their lowest, allowing the body to rest and repair. As dawn approaches, the hypothalamus in your brain releases corticotropin-releasing hormone (CRH), which signals the pituitary gland to produce adrenocorticotropic hormone (ACTH). ACTH then travels through the bloodstream to the adrenal glands, two small glands sitting on top of your kidneys, instructing them to ramp up cortisol production. This surge peaks roughly 30 to 60 minutes after waking. From this high point, cortisol is designed to taper gradually throughout the day, reaching its nadir around midnight, a rhythm known as the diurnal cortisol curve.

Several factors can influence your morning blood cortisol reading. Sleep quality is among the most significant: poor or fragmented sleep can blunt the awakening response, while chronic stress may initially push morning levels higher before eventually flattening them. Shift work, jet lag, medications such as corticosteroids, and recent illness can all affect results. For the most accurate reading, the sample should be taken in the morning, ideally between 8am and 9am, and before significant physical activity. If your morning cortisol is unexpectedly high or low, your GP can look at it alongside your clinical symptoms and, if needed, arrange further investigation such as a cortisol day curve or stimulation test.