Biomarker 101: Cortisol

Cortisol is a moving target. It follows a daily curve by design, which means the time on the collection slip matters as much as the number on the report.

What cortisol is

Cortisol is a steroid hormone produced by the adrenal glands, which sit above each kidney. It is often described as the stress hormone, though that undersells it considerably.

It helps mobilise energy by maintaining blood glucose, contributes to regulating blood pressure, moderates inflammation and immune activity, and participates in the metabolism of fats, proteins and carbohydrates. It is not something you would want less of in general — it is essential.

The daily rhythm

Cortisol is highest in the early morning, typically peaking around waking, then declines across the day to its lowest point overnight.

This is the single most important thing about interpreting it. A morning sample and an afternoon sample from the same healthy person will differ substantially, and neither is wrong. Blood tests are conventionally taken in the morning for exactly this reason, and a result without a collection time attached is difficult to read.

How it's controlled

Cortisol production runs on a feedback loop. The hypothalamus signals the pituitary, which releases ACTH, which prompts the adrenal glands to produce cortisol. Rising cortisol then feeds back to reduce the signal.

Because three organs are involved, an abnormal cortisol can originate at any point in that chain — which is why ACTH is often measured alongside it, and why establishing a cause requires more than a single measurement.

What shifts cortisol

Plenty of ordinary things move it. Acute stress and illness raise it. Exercise raises it. Pregnancy raises it. Shift work and disrupted sleep alter the rhythm itself, which can matter more than the absolute level. Some medications, including oral contraceptives and any form of steroid treatment, affect results directly.

Steroid medication is worth flagging specifically: taken over time it suppresses the body's own cortisol production, which changes how any result should be read. Your doctor needs to know about it.

When it's investigated properly

There are genuine conditions involving too much or too little cortisol, arising from the adrenal glands or the pituitary. They are uncommon, and they are not diagnosed from a single blood cortisol.

Assessing them requires dynamic testing — measurements taken after the system has been deliberately stimulated or suppressed — along with timed samples and sometimes urine or saliva collection over 24 hours. That is specialist investigation, arranged and interpreted by an endocrinologist.

What a single cortisol won't tell you

• It doesn't measure your stress levels. Cortisol responds to stress, but a number does not quantify how stressed you are.
• Timing dominates. Without knowing when the sample was taken, the result means very little.
• It doesn't diagnose anything alone. Adrenal and pituitary conditions require dynamic testing.
• "Adrenal fatigue" is not a recognised diagnosis. The term is widely used online; it has no accepted clinical definition and is not supported by evidence.

The bottom line

Cortisol is essential, follows a predictable daily curve, and responds to a great many ordinary circumstances. A single morning measurement gives a reasonable snapshot — and answering anything more specific requires testing designed for the question.