Hair vs blood cortisol test: which tells you more?

A blood test measures how much cortisol is in your blood at the moment the sample is taken. A hair test measures the cortisol laid down in your hair as it grew, which gives an average over roughly the past three months.¹ So the two answer different questions. If you want to know whether your cortisol has been running high or low over recent months, a hair test is the most accurate way to measure it from a single sample. If a doctor suspects adrenal insufficiency, a morning blood test, followed where needed by a Synacthen test, is the right tool, and hair has no role in that diagnosis.²³

What each test actually measures

Most of the cortisol in your blood is not free to act. About 90 per cent is bound to a carrier protein called corticosteroid-binding globulin (CBG), about 4 to 6 per cent to other proteins such as albumin, and only about 4 to 6 per cent circulates free.⁴ A standard blood test measures the total, bound and free together, reported in nmol/L or µg/dL (1 µg/dL is 27.59 nmol/L).⁴⁵ That is one reason blood results move when the level of carrier protein changes, even when the active hormone does not.

A hair test measures the cortisol stored in the hair shaft, reported in picograms per milligram of hair (pg/mg). Scalp hair grows about 1 cm a month, so the 3 cm nearest the scalp reflect roughly three months.¹ In a study that compared hair with a month of saliva samples, the researchers concluded that hair reliably estimates long-term free cortisol production.⁶ Our guide to how hair cortisol testing works has the detail. The table sets out the main differences between hair, blood, saliva and urine.

Why the time of day matters so much for blood

Cortisol follows a strong daily rhythm. It starts rising from about 4am, peaks within three hours of waking and falls through the day, with pulses every one to two hours on top.⁴ In healthy adults studied around the clock, blood cortisol peaked at about 8.30am at around 15.5 µg/dL (about 430 nmol/L) and fell below 2 µg/dL (about 55 nmol/L) shortly after midnight.⁷⁵ The same person can have a high or a low result depending only on when the sample was taken.

That is why laboratories ask for a morning sample and write their ranges for a time window. One NHS laboratory uses 133 to 537 nmol/L for samples taken between 6am and 10am, and below 50 nmol/L at midnight.⁸ A US reference gives 5 to 25 µg/dL (138 to 690 nmol/L) at 8am.⁹ Another NHS laboratory states that a random sample taken away from 9am cannot be interpreted.¹⁰ Shift work makes this harder still: NICE advises getting specialist advice before testing shift workers, because it is uncertain when to test.¹¹ Home finger-prick kits follow the same rule and ask for the sample before 10am.¹²¹³

Hair has no timing rules. Hair cortisol does not change with the time of day the sample is cut, and a bout of heavy sweating does not change it either.¹⁴

The needle, the journey and a stressful morning

Cortisol also rises with stress, and having blood taken can be stressful in itself. A review of the research suggested that venepuncture (taking blood from a vein) triggers a cortisol response about 30 per cent of the time. In the authors' own study of 18 healthy men, 47 per cent responded, and those who responded once were very likely to respond again.¹⁵

Other things on the morning of the test push in the same direction. Psychological stress raises cortisol, most of all when a situation feels uncontrollable or you feel judged,¹⁶ and exercise at moderate to high intensity raises it too.¹⁷ A rushed journey to the clinic or a hard workout beforehand can leave a single morning value reflecting the morning more than your usual level. A hair sample is barely touched by any of this: a stressful day is one of about ninety in a 3 cm sample, and the stress of the test cannot reach hair that has already grown.¹

Oral oestrogen, the pill and pregnancy

Because a blood test measures total cortisol, anything that raises CBG raises the result. Oral oestrogen, found in the combined contraceptive pill and some HRT, does exactly that, as do the high oestrogen levels of pregnancy.⁴ In a study of 37 women, those taking oral oestrogen had 67 per cent higher total blood cortisol than women taking none (660.9 against 395.4 nmol/L on average). Women using oestrogen through the skin had no rise, and free cortisol in saliva was similar in all three groups.¹⁸

NICE advises stopping oral oestrogen for six weeks before serum cortisol is measured, because otherwise the result will read falsely high.² The same effect matters for the dexamethasone suppression test used to look for Cushing's syndrome, which gives false-positive results in about half of women on the combined pill.¹⁹ Do not stop contraception or HRT for a test without talking to the doctor who arranged it.

Hair is far less affected. Hair studies of the combined pill have found at most a small and inconsistent effect, and in a 2026 study, users and non-users had similar hair cortisol (medians of 2.22 and 2.59 pg/mg).²⁰

Different laboratories, different numbers

Blood cortisol is usually measured by automated immunoassay, and the methods do not agree perfectly. Five automated immunoassays read 1.08 to 1.36 times higher than a mass spectrometry reference method, and assay-specific normal data are lacking.⁴ When one NHS laboratory changed its method, it warned users that results would read about 20 per cent lower from that date.⁸ Immunoassays can also cross-react with prednisolone, a common steroid medicine.⁴ NICE's decision thresholds for morning cortisol apply only to modern immunoassays.²

Hair has the same problem in a bigger form. There is no single normal range, and healthy values differ more than tenfold between methods, from about 0.8 to 8 pg/mg by mass spectrometry to 17.7 to 153.2 pg/mg by an early immunoassay.²¹²² Whichever test you have, compare the result only with the range on your own report. See hair cortisol results explained.

Which is more accurate for long-term cortisol?

A well-timed blood test is accurate for what it measures: the cortisol in your blood at that point. The trouble is that a moment is a poor guide to your usual level. Hair answers the long-term question directly. In a study of 17 adults who collected saliva three times a day for 30 days, the centimetre of hair nearest the scalp correlated with their 30-day saliva average at r = 0.61.⁶

Hair is also consistent. In the same study, 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.⁶ In repeated measurements months apart, a stable personal component explained 59 to 82 per cent of the variation in hair cortisol.²³ Blood was not part of those studies, but a single blood sample is subject to the same daily rhythm and stress responses as saliva, plus the needle and carrier proteins. That is why hair is the most accurate way to measure long-term cortisol from a single sample. There is no perfect yardstick for cortisol over months, so no test can claim to be exact. Read more in is hair cortisol testing accurate?

What a blood test is the right choice for

Blood tests are how doctors diagnose adrenal insufficiency, including Addison's disease. NICE advises an 8 to 9am serum cortisol when adrenal insufficiency is suspected, with these thresholds for people aged 16 and over:²

• Below 150 nmol/L: the person may have adrenal insufficiency; refer to an endocrinologist.
• 150 to 300 nmol/L: uncertain; consider repeating the test.
• Above 300 nmol/L: adrenal insufficiency is very unlikely.

The diagnosis is confirmed with a Synacthen (ACTH stimulation) test. Blood cortisol is measured before and 30 or 60 minutes after an injection of synthetic ACTH, and a peak below 500 nmol/L is diagnostic of primary adrenal insufficiency, although exact cut-offs depend on the local assay.¹¹ The Endocrine Society calls the short Synacthen test the gold standard.³ Blood is also used in the overnight dexamethasone suppression test for Cushing's syndrome, where a cortisol above 50 nmol/L (1.8 µg/dL) the next morning is the positive cut-off.¹⁹ If you have had a steroid injection into a muscle or joint, NICE advises waiting four weeks before testing.²

Hair cortisol is not part of these guidelines. If you have symptoms such as extreme tiredness, weight loss, dizziness on standing or patches of darker skin, see a doctor rather than relying on any home test.²⁴ Our guides to the blood cortisol test, the Synacthen test and testing for Addison's explain what to expect.

Getting each test

Blood cortisol tests are sold direct in the UK and the US, either as finger-prick kits you post to a laboratory or as a venous draw at a clinic, sometimes for an extra fee. The table lists tests from major providers, with prices as of October 2026.

Hair tests are sold as home kits. You cut a small sample from the back of the head whenever suits you and post it, with no needle and no appointment. If your question is about the last few months rather than one morning, a hair test answers it directly. For the other comparisons, see hair vs saliva and hair vs urine.