Cortisol: what a single morning test can and cannot tell you about stress

Cortisol is often called the stress hormone, so it is natural to hope a blood test will show how stressed you are. It will not, at least not in the way people expect. A single morning cortisol answers a narrower but still useful question: are your adrenal glands making cortisol at roughly the right level at the time of day they should be?

What cortisol does

Cortisol is made by the adrenal glands, which sit on top of the kidneys. The pituitary gland in the brain controls it through a hormone called ACTH. Cortisol helps regulate blood sugar, blood pressure, the immune system and your response to illness and injury. You cannot live without it, and too much of it over a long time is harmful.

A hormone with a daily rhythm

Cortisol follows a strong daily cycle. It climbs in the early hours, peaks shortly after you wake, then falls through the day to its lowest point around midnight. The difference between morning and evening is large, so the same number can be normal at 8am and high at 8pm.

That is why only a morning sample, taken between about 7 and 9am, can be compared with the lab's reference range. A sample taken later in the day tends to read lower and can wrongly look low. If you work night shifts, your rhythm is shifted and a standard morning test is hard to interpret, so mention it to whoever reads your results.

What a morning cortisol can show

The main value of a morning cortisol is as a screening test for adrenal insufficiency, a condition where the body does not make enough cortisol. A clearly normal morning level makes this unlikely. A low level needs repeating and, usually, a specific stimulation test arranged by a doctor.

Adrenal insufficiency can cause tiredness, weight loss, dizziness on standing, nausea, salt craving and darkening of the skin. Someone with a low morning cortisol and these symptoms, especially vomiting or feeling faint, should see a doctor urgently. LifePrint's report marks results like this with a clear urgency level.

What it cannot show

• How stressed you are. Everyday stress causes short spikes in cortisol that a single test cannot capture. A normal result does not mean you are not stressed, and a high one does not prove you are.
• Whether you have too much cortisol over time. A high morning result is common and usually harmless. Checking for Cushing's syndrome, a condition of long-term cortisol excess, needs different tests, such as a late-night saliva test, a 24 hour urine collection or a suppression test, arranged by a doctor.
• “Adrenal fatigue”. This is not a recognised medical diagnosis, and the Endocrine Society has said there is no scientific evidence for it. Ongoing exhaustion deserves a proper look for real causes, such as low iron, thyroid problems, poor sleep, depression or other illness.

Stress still matters

Long-term stress affects sleep, blood pressure, blood sugar and mood, and the effects often show up elsewhere in your results. A rising HbA1c, triglycerides or blood pressure may say more about the toll of stress than cortisol does. The Complete panel gives you that wider picture in one draw.

Getting a reliable result

• Book a draw between 7 and 9am.
• Keep taking prescribed medicines, including steroids, unless your doctor says otherwise. Never stop steroid medicine suddenly, because this can be dangerous.
• Avoid testing during an acute illness.
• Note the time of your draw and any steroid or hormone medicines you use.

If your cortisol is flagged as high or low, a single result is a starting point, not an answer. Discuss it with a registered healthcare practitioner, who can decide whether further testing is needed.