The best time for a cortisol blood test is early in the morning, because cortisol peaks soon after waking and falls through the day. At Blood London that means a weekday visit at 09:00, when our doors open at 117a Harley Street. The test is £75 plus the £50 blood draw, and the result is back in 1 day. Walk in or book online, and choose the first appointment of the day.
Cortisol across the day
Cortisol is a hormone made by the adrenal glands, two small glands that sit above the kidneys. It helps the body manage energy, blood pressure and the response to stress.
The level climbs through the early hours, peaks soon after waking, then drifts down through the afternoon and evening to its lowest point around midnight. Think of it as a tide that is high at breakfast and low at bedtime. A reading of the same size can mean one thing at 09:00 and something different at 16:00, so the doctor reading it needs to know when the sample was taken.
"Cortisol is highest in the morning, so I ask people to come in early, ideally before ten. A random afternoon result is hard to read."
Dr Mohammad Bakhtiar, Clinical Lead, GMC 4694470
What the test looks into
Doctors measure cortisol when they want to know whether the adrenal glands are making too little or too much of it. Too little is seen in Addison's disease, and too much in Cushing's syndrome. A morning blood test is the usual first test when too little cortisol is the question, and a low result is usually followed by a stimulation (Synacthen) test. When too much cortisol is the question, a doctor usually chooses an overnight suppression test, a late-night saliva test or a 24-hour urine collection instead of a single morning sample.
The evening before
The test page lists no special preparation, so an ordinary evening is fine. Eat, drink water and sleep as you usually do. Our guide to fasting before a blood test explains when fasting matters for other tests.
Three things are worth telling us when you book or arrive:
- Any steroid treatment you use. Steroid tablets, creams and inhalers can change the result, and a morning dose taken before the test can make it hard to read.
- Oral oestrogen, such as the combined pill or HRT tablets, which can raise a cortisol reading.
- Any biotin supplement. High-dose biotin can interfere with some hormone tests.
Ask the doctor who prescribes it, or book the optional doctor review before your test, about timing that morning's dose, and stop nothing without medical advice.
The morning of the test
Our doors open at 09:00 on weekdays, so a weekday 09:00 visit gives the earliest morning sample we can take at 117a Harley Street. Laboratory guidance commonly puts the best window at around 8 to 9am (Lab Tests Online UK), and 09:00 sits at the end of it. At weekends we open at 10:00, so the sample is a little later in the morning. Arrive when the doors open.
If you work nights or shifts, tell us your sleep pattern. Your daily rhythm may sit later than most people's, and the doctor reading the result needs to know.
The visit
A phlebotomist takes one blood sample from a vein in your arm. The £50 draw is charged once per visit, so any other blood tests you add share it.
At Blood London the sample reaches the lab the same day, and the report comes to you directly. Make a note of the time your blood was taken and give it to the doctor reading your result, so they can judge it against the daily curve. A weekend 10:00 sample is then read as a 10:00 sample.
The result
The cortisol result is ready in 1 day. Laboratory ranges are usually set for an early-morning sample, so the time of your draw matters when the result is read.
If you would like it explained, a doctor review can be added for £75. A GP at Medical Express Clinic in the same building (£150) can also go through it with you and arrange a Synacthen test if one is needed.
When a urine or stimulation test is used instead
A single blood sample shows cortisol at one moment. Sometimes a doctor wants a wider picture:
- Urine cortisol is £101.88, with results in 5 days. It suits cases where one blood sample is not enough, and it is usually collected over 24 hours. See the urine cortisol test.
- ACTH is the pituitary hormone that tells the adrenal glands to make cortisol. Doctors sometimes measure it alongside cortisol. See the ACTH blood test.
- A Synacthen stimulation test checks how the adrenal glands respond when given a prompt. It takes a baseline sample and a second sample about 30 minutes later, in a visit of 45 to 60 minutes. Our Synacthen test explainer walks through it, and the Synacthen stimulation test page shows how to enquire.
Blood or urine: the price in one place
| Test | Price | Result |
|---|---|---|
| Cortisol blood test | £75 plus the £50 draw (£125 for one blood test) | 1 day |
| Urine cortisol | £101.88 | 5 days |
The right choice depends on the question being asked, so if you are unsure, book the optional doctor review or a GP appointment first. Browse the full list of blood tests in London to add other tests to the same visit.
Can I have a cortisol test at the weekend?
Yes. Saturday and Sunday walk-ins run 10:00 to 14:00. Arrive at 10:00 for the earliest sample; note the time of the draw for the doctor reading your result. A weekday 09:00 visit remains the best match for the usual morning window.
Book your cortisol blood test or walk in at 117a Harley Street, Monday to Friday 09:00 to 18:00 and Saturday and Sunday 10:00 to 14:00. At Blood London, the earlier you arrive, the easier your result is to read.