The cortisol blood test: what it measures, and why timing is everything.
A cortisol blood test measures the level of cortisol, the body's main stress hormone, in a sample of blood. Because cortisol rises and falls on a daily rhythm, the time the sample is taken changes the result more than almost any other factor, which is why the standard collection is early in the morning.
What cortisol does
Cortisol is made by the adrenal glands under instruction from the pituitary gland in the brain. It is often described as the stress hormone, and it does surge when you are frightened, ill or in pain. But its everyday job is broader: it helps regulate blood sugar, blood pressure, the immune response, salt and water balance, and the way the body uses fat and protein. You cannot live without it.
The pituitary sends a signal called ACTH; the adrenals respond with cortisol; and when cortisol is high enough the brain dials the signal back down. Doctors call this loop the HPA axis. A cortisol test is a snapshot of that loop at one moment.
The daily rhythm, and why the cortisol blood test is a morning test
In a healthy person cortisol is at its highest shortly after waking and falls steadily across the day, reaching its lowest point around midnight. The difference between the morning peak and the evening trough is large. A result that would be perfectly normal at 8am might look high at 10pm, and a result that is normal at 10pm might look worryingly low at 8am.
That is why laboratories and guidelines specify a morning collection, usually between about 8am and 9am, for a standard cortisol blood test. The reference range printed on the report is a morning range. If the sample is taken at another time, the number is hard to interpret and the doctor may ask for it to be repeated.
- Book the earliest slot you can. Collection centres open early for exactly this reason.
- Keep to your usual sleep pattern. Shift work and jet lag shift the rhythm, and the doctor needs to know.
- Mention any steroid medicines, including inhalers, creams, joint injections and tablets. They can suppress or mimic cortisol and are one of the most common reasons for an odd result.
- Note the pill. Oestrogen raises the protein that carries cortisol in blood, so total cortisol reads higher on the combined pill even when the active hormone is unchanged.
Why doctors order it
Cortisol is not part of every hormone screen. On its own, it is ordered when there is a specific question:
- Could cortisol be too low? Persistent fatigue, weight loss, low blood pressure, dizziness on standing, salt cravings and darkening of the skin can point toward under-active adrenals. This is uncommon but important, and a morning cortisol is the first step. If these symptoms come with severe vomiting, confusion, fainting or collapse, that is an emergency: call 000 rather than waiting for a test.
- Could cortisol be too high? Weight gain concentrated around the trunk and face, easy bruising, thin skin, purple stretch marks, muscle weakness and high blood pressure raise the question of cortisol excess. Again uncommon, and again a morning cortisol is where the doctor starts, though it is rarely where they finish.
- Making sense of other results. Occasionally cortisol is added to a wider panel when fatigue, weight change or irregular cycles have no clear explanation and the doctor wants to exclude an adrenal cause.
What a single result can and cannot tell you
A morning cortisol that sits comfortably in the normal range makes a serious adrenal problem less likely. That is genuinely useful reassurance. What it cannot do is measure "stress" in the everyday sense, and it cannot confirm or rule out conditions marketed online as adrenal fatigue, which is not a recognised diagnosis in Australian or international guidelines.
A result outside the range does not settle anything either. Because cortisol responds to illness, poor sleep, pain, medicines and the anxiety of the blood draw itself, a one-off result that is a little high or a little low is often repeated first. If it stays abnormal, the doctor moves on to tests that look at the rhythm and the feedback loop rather than a single point.
The tests that follow a cortisol blood test
You may hear about these if your doctor is investigating further. They are ordered by a treating doctor, not requested off the shelf.
- Late-night salivary cortisol. Checks whether the evening trough is present. Useful when cortisol excess is the question.
- 24-hour urine cortisol. Measures total output across a day.
- Suppression and stimulation tests. The doctor gives a medicine and measures how cortisol responds, to see whether the feedback loop is working. These are done in a clinic setting.
- ACTH. The pituitary signal, measured alongside cortisol to work out where a problem sits.
Cortisol and the other hormones in your panel
Cortisol does not exist in isolation. High cortisol can suppress ovulation and disrupt cycles, and it pushes blood sugar up, which is one reason it sometimes sits near glucose and insulin on a request form. Thyroid function is often checked at the same time, because low thyroid and low adrenal function can look alike and occasionally occur together. And if a woman has androgen-type symptoms, the adrenal picture is completed with DHEA-S, which reflects a different adrenal output from the same gland.
Reference ranges for cortisol vary between laboratories and depend on the time printed on the collection tube, so we do not quote figures. The report will state the range it used; read your result against that, with your doctor.
Common questions
Do I need to fast for a cortisol blood test?
Not for cortisol itself. If glucose or lipids are on the same form you will need to fast for those. Water is fine. Try to avoid strenuous exercise before the collection, as it can push cortisol up temporarily.
Can stress from the blood test itself raise my cortisol?
A little, yes, particularly if you are anxious about needles. It is one reason a mildly raised result is often repeated rather than acted on. Arriving with time to spare and sitting quietly for a few minutes first helps.
Can I do a home cortisol test?
Saliva collections can be done at home when a doctor requests them, usually to check the late-night level. They complement the morning blood test rather than replace it. Home collections are not diagnostic on their own; a doctor puts them together with the rest of the picture.
Will Medicare rebate a cortisol test?
When a GP orders it for a clinical reason, usually yes. A consumer-initiated test is self-pay and not rebated. If you bring your own doctor's request to KnowLuna you pay the test-only price; otherwise a KnowLuna women's health doctor can request it and go through the result in a video consult.
General information only, not medical advice. If your symptoms are severe or sudden, see a doctor promptly.