Normal plasma cortisol levels

Plasma cortisol levels are an important indicator for assessing adrenal cortex function and the activity of the hypothalamic-pituitary-adrenal axis. The normal reference range for plasma cortisol is not a fixed value, but rather varies throughout the day. Specifically, there are mainly the following patterns:

1. Morning peak: Plasma cortisol levels are highest between 8:00 AM and 10:00 AM, with a normal range of 138–635 nmol/L.

2. Afternoon decline: As time progresses, plasma cortisol levels gradually decrease, reaching a normal range of 83–441 nmol/L between 4:00 PM and 5:00 PM.

3. Nocturnal nadir: By 10:00 PM to 2:00 AM, plasma cortisol levels reach their lowest point.

Therefore, plasma cortisol levels exhibit a clear circadian rhythm, typically peaking in the early morning and gradually declining thereafter to reach the lowest levels in the evening. When interpreting test results, the exact time of sample collection must be taken into account. Abnormal cortisol levels may indicate various pathological conditions, including Cushing's disease and adrenal insufficiency, and should be evaluated in conjunction with clinical symptoms and other diagnostic findings.