Hair cortisol test: how it works, how accurate it is and what it can tell you

A hair cortisol test measures how much cortisol, the body's main stress hormone, has built up in a small sample of hair cut close to the scalp. Hair grows about a centimetre a month and takes up cortisol as it grows, so the 3 cm nearest the scalp hold a record of roughly the past three months.¹ One sample gives an average over all of those days rather than a reading from a single moment. That makes a hair test the most accurate way to measure your long-term cortisol from a single sample, which is usually the real question for anyone wondering whether stress, tiredness, poor sleep or weight changes have a hormonal side.

How cortisol gets into your hair

Substances reach a hair mainly from the blood supply around the follicle while the hair is forming, with smaller contributions from sweat, sebum and the outside world once it has grown.¹ The follicle itself can also make a little cortisol.² Exactly how much comes from each route is still being studied, but once cortisol is locked into the hair shaft it stays there, moving outwards from the scalp as the hair grows.

Scalp hair grows 0.6 to 1.4 cm a month, about 1 cm on average.¹ So the first centimetre from the scalp reflects roughly the most recent month, the second centimetre the month before, and so on. There is a delay at the newest end: hair takes about two weeks to reach the surface and scissors leave a short stub behind, so the centimetre nearest the scalp reflects hair formed roughly 1.3 to 2.2 months before the cut.³ Hair further out has been washed and exposed to sunlight for longer, and its cortisol content falls from one segment to the next, which is why laboratories concentrate on the 3 cm nearest the scalp.⁴

In the laboratory the hair is washed, dried, finely cut or ground, and the cortisol is extracted and measured, by immunoassay or by liquid chromatography with tandem mass spectrometry (LC-MS/MS). In the first international comparison of laboratories, the two LC-MS/MS methods gave practically identical results and the immunoassays correlated closely with them.⁵ Results are reported in picograms of cortisol per milligram of hair (pg/mg).

Why hair is the most accurate measure of long-term cortisol

Cortisol is never still. In healthy people it peaks at around 8.30 in the morning and falls to its lowest point around midnight, with smaller pulses every hour or two in between.⁶ On top of that rhythm it jumps with stress, exercise, food and poor sleep. Even having blood taken can do it: venepuncture triggers a cortisol response in somewhere between about 30 and 47 per cent of people.⁷ A blood or saliva test captures one point on that moving line.

That is a problem if your question is about the long term. In a classic study of salivary cortisol taken at the same time of day six weeks apart, about 75 per cent of the variation reflected the situation on the day and only about 21 per cent the person's stable level.⁸ Researchers estimate that you need saliva samples on at least three separate days just to get a reliable average, and up to ten days to describe the daily slope.⁹ A 24-hour urine collection smooths out the hours of one day, but in people with Cushing's disease it still varied by about 52 per cent from one collection to the next.¹⁰

Hair does the averaging for you. 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 salivary average at r = 0.61.¹¹ And because hair reflects months rather than moments, it is consistent: 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.¹¹ In repeated measurements over months, a stable personal component explained 59 to 82 per cent of the variation in hair cortisol.¹² Hair cortisol does not change with the time of day the sample is cut, or with a bout of sweating.¹³

A useful comparison is the HbA1c test for blood sugar: one measurement that reflects sustained exposure over recent months rather than the reading at the moment of the test. The comparison has limits, because HbA1c has standardised methods and agreed cut-offs that hair cortisol does not yet have, but the principle is the same.

Hair compared with blood, saliva and urine

Each type of test answers a different question. Blood and saliva tell you what your cortisol is doing at the moment of the sample. A 24-hour urine collection gives one day's total. Hair gives the average over months. If you want to know whether your cortisol has been running high or low for a while, hair answers that directly. The table sets out the differences, including where doctors still rely on the other tests.

There is one more practical advantage. Oral oestrogen, in many contraceptive pills and some HRT, raises the protein that carries cortisol in the blood, and in one study women taking it had 67 per cent higher total blood cortisol than women who were not.¹⁴ Hair studies of the combined pill have found at most a small and inconsistent effect.¹⁵ For more detail, see hair vs blood, hair vs saliva and hair vs urine.

What a hair cortisol test can show

Because it captures sustained exposure, hair cortisol has become a standard tool in stress research. A meta-analysis of 66 studies and more than 10,000 people found hair cortisol was 22 per cent higher in groups exposed to chronic stress, rising to 43 per cent when the stress was still going on.¹⁶ The same analysis found links with body mass index, waist-to-hip ratio and blood pressure.¹⁶

• Long-term stress. Whether a period of sustained pressure has shown up in your cortisol, not just in how you feel. Hair cortisol does not track self-reported stress consistently, which is part of why measuring it is informative.¹⁶
• Weight. In 2,527 adults aged 54 to 87, hair cortisol was higher in people with obesity and a larger waist, and was linked with obesity that persisted over four years, although the correlations were small.¹⁷
• Heart health. In a meta-analysis of eight studies, people with cardiovascular disease had higher hair cortisol than people without.¹⁸ These are associations, not proof that cortisol is the cause.
• Change over time. Because results are consistent, testing again after three to six months shows whether changes to sleep, work or treatment have moved your levels. Longer hair can be cut into month-by-month segments to show a timeline. See segmental analysis.
• Clinical research. In a study of 43 people with Cushing's syndrome, a single hair sample identified the condition with 93 per cent sensitivity and 90 per cent specificity, which the authors judged similar to the standard first-line tests.¹⁹ Segmented hair has also traced the ups and downs of cyclic Cushing's, which single tests can miss.²⁰

How to take a hair sample

There are no timing rules. You, or someone you live with, cut a small lock of hair from the back of the head, just below the crown, as close to the scalp as possible. That area is used because results vary least between neighbouring samples there.²¹ The kit explains how much to take and how to mark which end was nearest the scalp. Hair needs no fridge or freezer, but post it promptly: cortisol slowly declines in stored hair, and researchers recommend analysis within three to six months.²²

The cut is small and hidden by the hair around it. Body hair is not a reliable substitute for scalp hair, so if your head hair is very short, ask the laboratory before ordering.²³ Read the full guide to taking a hair sample.

What can affect the result

A few things can shift the cortisol measured in hair, and a good laboratory will ask about them.¹⁶

• Hair treatments. In the Whitehall II study, people who dyed their hair had 12.5 per cent lower hair cortisol.²⁴ Heat treatments have been linked with lower values too. See testing dyed or treated hair.
• Sunlight. In an experiment, 40 hours of natural sunlight lowered hair cortisol by a median of 54 per cent, which is another reason the newest hair is used.²⁵
• Steroid medicines. Tablets, inhalers, nasal sprays, creams and injections can raise or lower the result, depending on the medicine.²⁶ Tell the laboratory exactly what you use.
• Age and sex. Men tend to have somewhat higher hair cortisol than women, and levels can differ with age, so results are most useful read against a comparable group.¹⁶

What a hair cortisol test is not for

A hair test measures your long-term cortisol. It does not diagnose or rule out a medical condition on its own. Cushing's syndrome is diagnosed with late-night saliva, 24-hour urine and dexamethasone suppression tests, repeated where needed.²⁷²⁸ Primary adrenal insufficiency, including Addison's disease, is diagnosed with blood tests, the standard being the short Synacthen (ACTH stimulation) test.²⁹ Hair cortisol is not part of those guidelines, and a 2024 expert review noted that it still needs validating outside research settings.³⁰

If you have symptoms that could mean an adrenal condition, such as unexplained weight loss, dizziness on standing, darkening skin, purple stretch marks or easy bruising, see a doctor rather than relying on a home test. Our guides to testing for Cushing's and testing for Addison's explain what to expect.

Understanding your result

Your result is a number in pg/mg, shown against the laboratory's own reference range. Ranges differ a great deal between methods: a recent LC-MS/MS study of 256 healthy adults set a range of 0.84 to 8.07 pg/mg, while an early immunoassay study found 17.7 to 153.2 pg/mg in healthy adults.³¹²¹ So compare your result only with the range on your own report, and with your own earlier results from the same laboratory.

A result above the range means your body has been exposed to more cortisol than most healthy adults over the months tested; a result below it means less. Either is a reason to look at what has been happening in that period and, if you have symptoms, to talk to a doctor. Read hair cortisol results explained for more.