Segmental hair cortisol analysis: a month-by-month cortisol timeline

Segmental hair cortisol analysis cuts a longer lock of hair into pieces, usually 1 cm or 3 cm long, and measures each one separately. Because hair grows outwards from the scalp at about 1 cm a month, each piece reflects a different stretch of time, and together they give a rough month-by-month timeline rather than a single average.¹ It has traced cortisol through late pregnancy and through the ups and downs of cyclic Cushing's syndrome.¹² It also has firm limits: the dates are approximate, and cortisol fades from older hair, so in everyday use a timeline is reliable for only about the last six months.¹³ For how a standard single-sample test works, see how hair cortisol testing works.

How segments map to months

Researchers commonly assume hair grows 1 cm a month, so the first 3 cm from the scalp represent roughly the past three months, the next 3 cm the three months before that, and so on.¹ Real growth runs from 0.6 to 1.4 cm a month, and a review of head hair described the range of growth rates as remarkably wide.⁴⁵ The uncertainty builds up the further you go from the scalp. The table shows our own arithmetic, from those published growth rates, for the months each 3 cm segment would cover.⁴

The table leaves out one more shift. New hair takes about two weeks to reach the surface of the scalp, and scissors leave a stub, about 0.8 cm according to a forensic review, so the first 1 cm segment typically reflects hair formed 1.3 to 2.2 months before the cut.⁵ The whole timeline is pushed back by that amount, and the most recent weeks are not in the sample at all. The hair sample calculator works through the months for a given length of hair.

The pregnancy study: hair as a calendar

The idea of hair as a "retrospective calendar" of cortisol comes from a 2009 study of 103 women who had just given birth, 19 mothers of toddlers and 20 controls.¹ In the new mothers, the 3 cm segment nearest the scalp, which grew during the third trimester when cortisol naturally rises, had twice the cortisol of the controls' hair.¹ It was an early demonstration that hair could pick out a known period of high cortisol, months after the event.

The same study also revealed the method's main limitation. Across all the women, 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.¹ The authors concluded that a retrospective calendar may be limited to about the past six months.¹

Washout: why older segments read lower

That steady fall is known as the washout effect. A second study, of long-term unemployed people, confirmed it in the third 3 cm segment and also put the practical look-back at about six months.³ A 2017 meta-analysis of 66 studies found the same decline from the first 3 cm segment to the second.⁶

Laboratory experiments point to the causes. Shampoo, hot water and ultraviolet light all removed cortisol from hair segments, and 40 hours of natural sunlight cut hair cortisol by a median of 54 per cent.⁷⁸ The older the hair, the more washing and sunlight it has been through. Testing dyed or treated hair covers these factors in more detail.

Washout changes how a timeline should be read. A gradual fall from the newest segment to the oldest is expected and does not mean your cortisol was higher recently. What carries information is a departure from that pattern: a segment that stands out from its neighbours, or a difference at the same distance from the scalp, between two groups of people or between two of your own tests taken months apart. That is why the pregnancy study compared the same segment in mothers and controls rather than reading the raw slope.

Cushing's syndrome and cyclic disease

Segmental analysis has its clearest clinical use in Cushing's syndrome, where cortisol can be many times higher than normal. In a 2010 study, six patients had a median of 679 ng/g against 116 ng/g in 32 controls (ng/g and pg/mg are the same number), and segment-by-segment results gave information for up to 18 months before sampling, following each patient's clinical course.⁹

Timelines matter most in cyclic Cushing's syndrome, in which episodes of high cortisol alternate with periods of normal secretion. The standard 24-hour urine and late-night saliva tests reflect at most a day, so they can be normal if taken between episodes.² In a 2012 study that included six people with suspected cyclic disease, hair timelines matched the clinical course.² Hair has also shown cortisol rising before illness was obvious: in two patients with ectopic Cushing's syndrome, caused by a tumour outside the pituitary gland, segments showed raised cortisol 3 and 6 months before the disease became clinically apparent.¹⁰ And in a 2026 case report, very high cortisol along 11 cm of a 12-year-old boy's hair supported the diagnosis when conventional tests had given contradictory results. A pituitary tumour was confirmed 18 months after he first presented, most likely with a cyclical course.¹¹

These are specialist uses. The 2021 consensus guideline on Cushing's disease recommends repeated late-night saliva tests to identify cyclic disease, and it does not include hair.¹² In a 2026 Pituitary Society expert panel, 61.5 per cent of panellists chose hair cortisol as an emerging marker that warrants further research, but it was not endorsed as a recommended test.¹³ A segmental hair test cannot diagnose or rule out Cushing's syndrome. See testing for Cushing's syndrome.

What segmental results can and cannot tell you

• Can show whether cortisol was broadly higher or lower in one stretch of months than another, within about the last six months.¹³
• Can give a rough idea of whether a change came before or after a known event, such as a new job, a bereavement or a change of treatment, provided you allow for the uncertainty in growth.
• Cannot date a change to a particular week or month, because growth rates vary and the newest weeks are missing.⁵
• Cannot reliably compare cortisol across more than about six months in healthy people, because of washout.¹³
• Cannot diagnose a condition, or tell you why cortisol was high or low.

For most people the usual question, what your average cortisol has been over roughly the last three months, is answered by a single 3 cm sample. Segmental analysis adds something when the timing itself matters, for example when you want to see whether cortisol changed across a known event. Reading a single result is covered in hair cortisol results explained.

Practical points for a segmental test

You need longer hair: about 6 cm for two 3 cm segments and 9 cm for three, measured from the scalp.¹ Mark the scalp end clearly, as the kit explains, because the analysis depends on knowing which end is which. Cut from the back of the head just below the crown, as close to the scalp as possible, where results vary least: in one study cortisol varied by 15.6 per cent between samples from that area, against 30.5 per cent across the whole head.¹⁴ Tell the laboratory about dye and heat treatments, which are linked with lower values, and about any steroid medicines.¹⁵¹⁶ Post the sample promptly, because cortisol declines slowly in stored hair and analysis within three to six months is recommended.¹⁷ Full instructions are in how to take a hair sample.