How hair cortisol testing works: from the follicle to the laboratory

A hair cortisol test works because hair keeps a record. While a hair is forming in the follicle beneath the skin, cortisol from the surrounding blood supply is built into it, and once it is part of the hair shaft it moves out from the scalp as the hair grows.¹ Scalp hair grows about 1 cm a month, so a laboratory that measures the 3 cm nearest the scalp gets an average of roughly the past three months.¹ This guide goes through each step in more depth: how cortisol gets in, how growth rate sets the time window, why older hair reads lower, and how laboratories turn a lock of hair into a number. For the overview, start with the hair cortisol test.

How cortisol gets into a growing hair

Researchers describe hair as having several sources, which they call a multicompartment model. Substances arrive from the blood while the hair is forming, from sweat and sebum (the skin's natural oil) after it has formed, and from the outside world once it has emerged from the skin.¹ The same model is used for drugs, and a recent review applies it to cortisol as well.¹

Two findings complicate the picture. Human scalp hair follicles kept alive in the laboratory make substantial amounts of cortisol themselves, and they respond to corticotrophin-releasing hormone, the brain's first stress signal, in a way that mirrors the body's own stress system (the HPA axis), including its feedback brake.² So some hair cortisol may be made locally. On the other hand, hair cortisol in healthy adults tracks cortisol in their saliva, which the authors of one study took as support for cortisol diffusing in from the blood.³

Sweat looks like a minor route. In two experiments, a treadmill session with 42 people and a sauna with 52, heavy sweating did not change hair cortisol, and hair cortisol was unrelated to the cortisol in sweat, the rate of sweating or the time of day of the examination.⁴ The authors concluded that sweat is unlikely to be a dominant source.⁴

The fair summary is that hair cortisol comes mainly from the circulation, with contributions from the follicle and the skin's surface still under study. It matters less than it might seem: whatever the exact mix, hair cortisol behaves the way a long-term measure should, as is hair cortisol testing accurate? explains.

How fast hair grows, and why the window is roughly three months

Scalp hair grows 0.6 to 1.4 cm a month, about 1 cm on average.¹ Growth differs between parts of the body, with age, sex and ethnicity, and from one person to the next.⁵ A review of head hair found an average of 1.06 cm a month at the back of the crown, the area used for cortisol testing, and described the spread of growth rates as remarkably wide.⁶

That is why the window is "roughly three months" rather than exactly three. At 1.4 cm a month, 3 cm of hair covers a little over two months; at 0.6 cm a month it covers five.¹ (That is our arithmetic from the published growth rates.) Because a standard test does not measure your own growth rate, researchers work on the 1 cm a month convention, treating successive 3 cm segments as roughly the past three, six and nine months.⁷ If your hair grows noticeably slowly or quickly, the real window will be longer or shorter.

The lag at the scalp end: why the last few weeks are missing

A hair test does not reach right up to the day the sample is cut. New hair takes about two weeks to grow from the follicle to the surface of the scalp, and even a close cut leaves a stub: a forensic review puts the hair left on the scalp after cutting at about 0.8 cm.⁶ Putting those together, the first centimetre of a sample typically reflects hair formed between about 1.3 and 2.2 months before it was cut.⁶

In practice, a stressful fortnight just before you take a sample will barely register, and a change you made last month will not show yet. If you want a test to capture a particular period, such as the months after a change in work or treatment, allow for this delay as well as for growth. The hair sample calculator works through the dates for you.

Segments, and why older hair reads lower

Because hair grows outwards, a longer sample can be cut into pieces and each one measured separately, giving a rough month-by-month timeline. This is segmental analysis. The catch is that cortisol does not stay put for ever. In the study that first described hair as a "retrospective calendar", cortisol fell steadily from the scalp end to the tip, dropping by 30 to 40 per cent from one 3 cm segment to the next over the four most recent segments, and segments older than a year all had similarly low levels.⁷ A second study confirmed this washout in the third 3 cm segment, and both concluded that looking back is limited to about six months.⁷⁸ A 2017 meta-analysis of 66 studies also found the decline from the first segment to the second.⁹

The likely reason is the wear hair goes through after it grows. In laboratory experiments, shampoo, hot water and ultraviolet light all removed cortisol from hair segments.¹⁰ How that affects treated hair is covered in testing dyed or treated hair, and how timelines are read, including in Cushing's syndrome, in segmental hair cortisol analysis.

What happens in the laboratory

Laboratories ask for hair from the back of the head, just below the crown (the vertex posterior), because results vary less there. In one study, cortisol varied by 30.5 per cent across samples from different parts of the head, but by 15.6 per cent between samples from that one area.¹¹ The section nearest the scalp is measured off, cleaned, dried and then either ground to a powder or finely minced; which of those works better is one of the method choices a 2019 review examined, along with how the cortisol is extracted and measured.¹²

The measurement itself uses one of two families of method.

The first hair cortisol methods adapted ELISA (enzyme-linked immunosorbent assay) kits designed for saliva.¹¹ Liquid chromatography with tandem mass spectrometry, LC-MS/MS, is the more specific approach. The method above had run-to-run variation of 3.7 to 9.1 per cent and could quantify cortisol down to 0.1 pg/mg or less.¹³ A clinical LC-MS/MS method could detect cortisol below 1.3 pg/mg and varied by less than 16 per cent between runs over nine months.¹⁶

Do different laboratories agree?

Broadly, they rank people the same way, but they do not produce the same numbers. In the first international round robin, the same hair samples were measured by four immunoassay methods and two LC-MS/MS methods. The immunoassays correlated highly with one another (r squared 0.92 to 0.97, where 1 would be perfect) and with LC-MS/MS (0.88 to 0.97), and the two LC-MS/MS methods gave practically identical results.¹⁷ Correlation is not the same as agreement, though: the authors proposed correction factors to convert immunoassay results into LC-MS/MS equivalents.¹⁷

Kits and procedures matter too. Four laboratories in Canada, the USA, Germany and the Netherlands, each using a different ELISA kit, amount of hair and extraction method, reported values in healthy volunteers that differed by up to 2.3-fold: 46.1, 29.72, 20 and about 27 pg/mg.¹⁸ A 2025 systematic review that screened 11,716 publications found that collection procedures and average values varied significantly, that 92 per cent of studies used segments of up to 3 cm, and that many papers left out key method details. It proposed a reporting checklist.¹⁴

The practical lesson is simple. A hair cortisol result means most when it is read against the reference range of the laboratory that produced it, and compared with your own earlier results from the same laboratory and method.

Units: pg/mg and ng/g

Hair cortisol is reported as picograms of cortisol per milligram of hair (pg/mg). A picogram is a millionth of a microgram. Some papers use nanograms per gram (ng/g), which gives the same number: 1 ng/g equals 1 pg/mg.¹⁹¹¹ Because it is a weight of cortisol in a weight of hair, a hair result cannot be converted into the nmol/L used for blood, saliva and urine.

The method matters far more than the unit. An LC-MS/MS study of 256 healthy adults in northern Italy set a reference 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, a more than tenfold difference.¹⁵¹¹ That is why there is no single normal range for hair cortisol. Hair cortisol results explained covers how to read a report.

What the science has not settled

Hair cortisol is a young field compared with blood testing, and expert reviews are open about the unanswered questions.²⁰

• The mix of sources. How much hair cortisol comes from the blood, from the follicle itself and from sweat or sebum is not fully known.¹²⁴
• Your own growth rate. A test assumes about 1 cm a month, but your hair may grow faster or slower, which stretches or shrinks the window.⁶
• Standardisation. There is no standardised method, no agreed reference range and no guideline cut-off, and a 2024 expert review said hair cortisol still needs validating outside research settings.¹⁴¹⁵²¹

None of this undoes the core finding. Hair cortisol tracks the average of repeated saliva samples taken over the same month, and it is far more consistent from one test to the next than a single blood, saliva or urine sample.²² That is what makes it the most accurate way to measure long-term cortisol from a single sample.