How often should you test your cortisol?

If you are tracking your long-term cortisol with hair tests, testing every three to six months is usually enough. A standard 3 cm sample covers roughly three months, so a test every three months gives back-to-back windows, and a gap of six months still shows whether things have moved. Hair suits this because one person's result is stable from test to test unless something has genuinely changed. Blood, saliva and urine tests work differently: each captures a moment or a single day, so doctors repeat them simply to get one reliable answer, not to follow change over months.

Why hair results can be compared over time

A repeat test is only useful if the test itself is consistent, and hair is. In a study of 17 adults who gave hair, saliva and urine samples over a month, 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 averaged saliva and 0.59 to 0.68 for 24-hour urine.¹ In two further studies, one sampling people a year apart and the other three times at two-month intervals, repeat hair results correlated at r = 0.68 to 0.79, and a stable personal component explained 59 to 82 per cent of the variation.²

That stability is what makes a change worth noticing. If your result moves a long way between two tests taken the same way, it is more likely to reflect a real shift in your cortisol than chance. Small differences are another matter. There is no agreed threshold for a meaningful change, and one clinical laboratory method reported variation between runs of under 16 per cent over nine months, so a shift of a few per cent can simply be measurement noise.³

How often to retest

Scalp hair grows about 1 cm a month, although anything from 0.6 to 1.4 cm is normal.⁴ That sets the natural rhythm for repeat tests:

• Every three months gives roughly consecutive windows with little overlap, so nothing is missed. This suits someone following a period of change closely.
• Every six months shows whether a change has held, though the months between the two windows are not covered.
• More often than every three months adds little. A 3 cm sample cut six weeks after the last one shares about half its months with the previous test.
• Once is enough if you simply want to know where your long-term cortisol stands now.

Longer hair offers another option: one sample cut into month-by-month sections. Cortisol fades from hair further from the scalp, by 30 to 40 per cent from one 3 cm section to the next, so the original researchers suggested a look-back of about six months.⁵ In Cushing's syndrome, where levels are very high, 1 cm sections have traced cortisol up to 18 months back.⁶ See segmental hair cortisol analysis.

Keep each test the same

Comparisons only work if the tests are comparable. Use the same laboratory each time, because results differ hugely between methods: a recent mass spectrometry study set a healthy range of 0.84 to 8.07 pg/mg, while an early immunoassay study found 17.7 to 153.2 pg/mg.⁷⁸ Cut the same length from the same part of the head each time, as described in how to take a hair sample.

Note the season. In a review of 29 studies with 10,520 participants, 76 per cent reported seasonal differences, and most studies comparing people found lower values in winter and spring than in summer and autumn.⁹ In a South London sample, summer was linked to higher and winter to lower hair cortisol.¹⁰ If you compare tests a year apart, sampling at a similar time of year removes one source of difference.

Note any changes to your hair care and medicines, too. People who dyed their hair had 12.5 per cent lower hair cortisol in the Whitehall II study, and systemic steroid medicines were linked to higher values.¹¹ Starting to dye your hair between two tests could shift the second result for reasons that have nothing to do with stress.

Timing a test after a change

If you want to see the effect of a new job, the end of a stressful period, a treatment or a new sleep routine, timing matters. The newest hair is not in the sample: it takes about two weeks to reach the surface, and scissors leave a stub behind, so the centimetre nearest the cut reflects hair formed roughly 1.3 to 2.2 months earlier.¹² The forensic researchers who measured this delay advised waiting about eight weeks after a one-off event before collecting hair, so that the sample includes it.¹²

For a standard 3 cm sample to hold only hair grown since a change, you need about three months of new growth plus that delay, so cut it roughly four to five months after the change began. A sample cut at the start gives you the before picture; one cut four to five months later gives the after. The hair sample calculator works out the dates for you. Treat them as approximate, because growth rates vary from person to person.⁴

How quickly can hair cortisol change?

Only a few trials have used hair cortisol as their outcome, but they give a sense of the timescale:

• Meditation-based mental training. In a trial that randomised 362 adults, with hair data for 227, hair cortisol and cortisone fell consistently over the first three to six months of training, with no further fall by nine months. The effect was larger in people who practised more often, and it was unrelated to changes in how stressed people said they felt.¹³
• Aerobic exercise. In a year-long trial of 130 adults aged 26 to 58, 150 minutes a week of moderate to vigorous exercise lowered hair cortisol compared with a group given health information.¹⁴
• Treating insomnia. In a trial of digital cognitive behavioural therapy for insomnia in 74 nurses working shifts, hair cortisol fell by 1.8 pg/mg, or 44 per cent, within the treated group. This was a change within the group, not a comparison with the control group.¹⁵

A review of 14 intervention studies with 1,916 participants found mixed effects of relaxation, mental training and talking therapies, and higher hair cortisol at the start sometimes predicted a better response.¹⁶ So a change in habits does not guarantee a lower result. Where hair cortisol does change, it shows over months, which is why an interval of three to six months fits. For what the evidence says about lowering cortisol, see how to lower cortisol.

Why blood, saliva and urine tests need repeats

For single-moment tests the question is different. Each result is strongly shaped by the day it was taken: in a classic study of saliva samples 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.¹⁷ Researchers estimate that saliva needs sampling on at least three separate days for a reliable average, four to eight days for the total daily output and ten days for the daily slope.¹⁸ In people with Cushing's disease, 24-hour urine cortisol varied by 52 per cent from one collection to the next, and collecting more than two did not meaningfully reduce that.¹⁹

That is why repeats are built into the doctor's tests. The Endocrine Society recommends at least two 24-hour urine or late-night saliva measurements when Cushing's syndrome is suspected,²⁰ and a 2021 consensus advises two or three urine collections and at least two or three late-night saliva tests. After surgery for Cushing's disease, it recommends a late-night saliva test every year to check for recurrence.²¹ These repeats are about getting a dependable result now. They are not a way of measuring average cortisol over the months in between.

When retesting is not the answer

Repeating a home test is not a substitute for seeing a doctor. A hair test shows your long-term cortisol; it does not diagnose or rule out Cushing's syndrome, Addison's disease or any other condition. If you have symptoms that worry you, such as unexplained weight change, easy bruising, dizziness on standing or extreme tiredness, see a GP. Our guide to cortisol tests from your doctor explains what they can arrange.