Blood cortisol test: what it measures, how to prepare and what your result means

A blood cortisol test measures the cortisol in a sample of blood, usually taken from a vein in your arm between 8 and 9 in the morning.¹ It is the usual first test when a doctor suspects your adrenal glands are making too little cortisol, and blood cortisol is also what is measured in the dynamic tests used for both low and high cortisol. Because cortisol rises and falls through the day, the result describes one moment. The time of the test, the medicines you take and even how you feel about needles can all move the number, so a little preparation makes the result far easier to interpret.

What a blood cortisol test measures

Most laboratory blood tests measure total cortisol. In the bloodstream about 90 per cent of cortisol is bound to a carrier protein called corticosteroid-binding globulin (CBG), about 4 to 6 per cent is bound to other proteins such as albumin, and only about 4 to 6 per cent circulates free, which is the active form.² A blood test counts all of it. That matters because anything that changes the amount of carrier protein changes the result without changing the cortisol your body can actually use. In critically ill people with low albumin, for example, total cortisol was lower (435 against 633 nmol/L) while free cortisol was much the same (140 against 143 nmol/L).² Saliva and urine tests measure free cortisol instead, and are not affected by CBG.²

Doctors use a morning blood cortisol mainly to look for adrenal insufficiency, where the adrenal glands make too little cortisol.¹ It is not a screening test for Cushing's syndrome, where they make too much: the guidelines start with 24-hour urine, late-night saliva or the overnight dexamethasone suppression test.³ Blood cortisol is, though, what is measured at the end of the dexamethasone suppression test and during the Synacthen test.

Why the test is done at 8 to 9am

Cortisol follows a daily rhythm. It starts rising from about 4am, peaks within three hours of waking, then falls through the day to its lowest point around midnight, with smaller pulses every one to two hours.² In a healthy reference group of 33 people, serum cortisol peaked at 15.5 µg/dL at about 8.30 in the morning and fell below 2 µg/dL shortly after midnight.⁴ The same person can have a healthy result at 9am and a much lower one in the late afternoon.

That is why NICE advises an 8 to 9am serum cortisol when adrenal insufficiency is suspected,¹ and why a value taken at a random time is hard to interpret.² One NHS laboratory states plainly that a random (non-9am) cortisol, or one taken while the patient is on steroid treatment, cannot be interpreted.⁵ If you work shifts, the usual timing may not match your body clock. NICE's guidance for GPs advises getting specialist advice before testing shift workers, because it is uncertain when to test and the results can be hard to read.⁶ See cortisol and shift work.

How to prepare for a blood cortisol test

Book an early appointment, and tell whoever arranges the test about every medicine and supplement you take. The main things that can distort the result are these.

• Oral oestrogen. Many contraceptive pills and some HRT tablets contain oestrogen, which raises CBG and so gives a falsely high total cortisol; pregnancy does the same.² In one study, women taking oral oestrogen had 67 per cent higher total cortisol than women who were not, while oestrogen given through the skin had no such effect.⁷ NICE advises stopping oral oestrogen for 6 weeks before serum cortisol is measured.¹ Do this only if the doctor arranging your test advises it, and plan contraception with them.
• Steroid medicines. Glucocorticoids such as prednisolone and hydrocortisone can suppress your own cortisol, and some cross-react with the test itself.⁸² Inhaled, injected and skin steroids can also give a low morning result when the dose is at or above what the body would normally make, and NICE advises waiting 4 weeks after a steroid injection into a muscle or joint before testing.¹ Never stop a steroid on your own: stopping suddenly is itself a common cause of abnormal cortisol, and medicines should only be stopped after talking to your doctor.⁹
• Biotin. High-dose biotin supplements, which some people take for cosmetic reasons, can make some cortisol immunoassays read falsely high. Laboratory guidance says doses of 5 mg or more can interfere, while the amounts in ordinary multivitamins have not been reported to.¹⁰ After a 5 to 10 mg dose it advises waiting at least 8 hours before a blood test, and up to 72 hours for some assays and higher doses.¹⁰ Some providers simply ask you to stop biotin for 72 hours.¹¹
• Exercise and illness. You may be asked to avoid hard exercise the day before. Acute illness raises cortisol, so mention it if you are unwell.⁸ MedlinePlus's preparation advice does not include fasting, but follow any instructions you are given, especially if other tests are run on the same sample.⁸

What happens during the test, and why the needle matters

A nurse or phlebotomist takes a small sample from a vein, which takes a few minutes. Some private providers instead send a finger-prick kit to use at home in the morning, or arrange a draw at a clinic. The table lists examples.

Having blood taken can itself raise cortisol. A review of the research suggested that venepuncture triggers a cortisol response about 30 per cent of the time, and in the authors' own study of 18 healthy men, 47 per cent responded.¹² Those who responded once were highly likely to respond again.¹² Stress before the draw, such as a rushed journey, can push the result up too.¹¹ If you are anxious about needles, a slightly raised result may partly reflect the test itself, which is worth mentioning to the doctor reading it.

Normal ranges and what your result means

There is no single normal range. Each laboratory sets its own, for its own assay and time window, so the range printed on your report is the one that applies. These examples show how much they differ.

Part of the difference is the assay. Five automated immunoassays read 1.08 to 1.36 times higher than a mass spectrometry reference method, and assay-specific normal data are lacking.² When one NHS laboratory changed its method, it warned users that results would read about 20 per cent lower from that date.¹⁵ UK laboratories report in nmol/L and US ones often in µg/dL; to convert, multiply µg/dL by 27.59,¹⁶ or use our cortisol converter.

For suspected adrenal insufficiency in people aged 16 and over, NICE uses these thresholds for an 8 to 9am cortisol measured with a modern immunoassay.¹

• Below 150 nmol/L: adrenal insufficiency is possible; the person should be referred to an endocrinologist.
• 150 to 300 nmol/L: uncertain; consider repeating the test.
• Above 300 nmol/L: adrenal insufficiency is very unlikely.

NICE's guidance for GPs adds that a result below 100 nmol/L in someone with suspected Addison's disease makes adrenal insufficiency highly likely and calls for hospital admission.⁶ More generally, a high morning result can reflect Cushing's syndrome, an adrenal tumour, stress or acute illness, and a low one Addison's disease, a pituitary problem or steroid medicines.⁸ Either way, a single result is a starting point for further tests, not a diagnosis. See normal cortisol levels for more on what is typical.

What happens after an abnormal result

If your morning cortisol is low or borderline, the next step is usually a short Synacthen test, which checks whether your adrenal glands respond when stimulated. It is the standard test for confirming primary adrenal insufficiency, and a plasma ACTH level, which can be taken at the same time as the first sample, helps show whether the problem lies in the adrenal glands themselves.¹⁷ If cortisol excess is suspected, a doctor will arrange one or more of the first-line tests for Cushing's syndrome, with urine or saliva collected on at least two occasions.³ Our guides to the Synacthen test, the 24-hour urine test and testing for Cushing's explain what each involves.

Low cortisol can occasionally become an emergency. Vomiting, severe dizziness, confusion or drowsiness in someone with known or suspected adrenal insufficiency need urgent help, because they can signal an adrenal crisis.¹⁸ See low cortisol symptoms.

Can a blood test show your long-term cortisol?

Not well. A blood test shows one point on a line that moves with the hour, the day and the needle. That suits the morning and stimulation tests doctors rely on, but if your question is whether your cortisol has been running high or low over recent months, perhaps alongside stress, tiredness or poor sleep, a single sample says little about it.

For that question a hair test is the most consistent single-sample measure. Scalp hair grows about 1 cm a month and takes up cortisol as it grows, so a few centimetres near the scalp reflect roughly the past three months.¹⁹ In one study the same people's hair cortisol correlated at r = 0.84 from one month to the next, against 0.38 to 0.61 week to week for saliva and 0.59 to 0.68 for 24-hour urine.²⁰ Hair cortisol does not vary with the time of day the sample is cut.²¹ It does not diagnose or rule out adrenal insufficiency or Cushing's syndrome, which are diagnosed with the blood, urine and saliva tests described here and in the linked guides.¹⁷³ Read hair vs blood cortisol tests for the full comparison.