Hair vs 24-hour urine cortisol test: one day or three months?

A 24-hour urine test measures the free cortisol your kidneys pass over one full day. A hair test measures the cortisol built up in your hair over roughly the past three months.¹ The 24-hour urine collection is one of the first-line tests doctors use to look for Cushing's syndrome; hair is not.² For your long-term level, hair is the most accurate measure from a single sample, and it is much more consistent from one test to the next. In healthy people, though, the two tests agree only weakly to moderately, and this guide sets out what the studies found and why.

What a 24-hour urine test measures

For a 24-hour urinary free cortisol (UFC) test, you collect all the urine you pass over a full day and night in a container from the laboratory. The laboratory measures the free cortisol in it, the fraction that is not attached to carrier proteins. Some tests also measure creatinine, a waste product passed at a steady rate, to check that the collection is complete.³ Because UFC measures free cortisol, it is not affected by the rise in carrier protein that oral oestrogen causes in blood tests.⁴

Normal ranges depend on the laboratory and method. One US reference gives below 45 µg (124 nmol) per 24 hours for women and below 60 µg (165 nmol) for men,⁵ while a clinical trial used 30 to 145 nmol per 24 hours.⁶ Sex, body mass index, age, the volume of urine and how much salt you eat all influence the result,⁷ and UFC can read falsely low when kidney function is reduced, with a creatinine clearance below 60 mL/min.² Our guide to the urine cortisol test covers the collection step by step.

How much a 24-hour urine result varies

A 24-hour collection smooths out the hours of one day, but not the differences between days. In 152 people with Cushing's disease who gave more than 600 collections between them, UFC varied by about 52 per cent from one collection to the next in the same person. Collecting more than two samples did not meaningfully reduce that variability, and UFC did not track how severe the disease was.⁶ The 2021 consensus on Cushing's disease says random variability in UFC can reach 50 per cent and advises two or three collections;⁷ the Endocrine Society recommends at least two.²

In healthy people, repeated UFC is steadier, but still less consistent than hair. In a study that measured both in the same adults, 24-hour urine correlated at r = 0.59 to 0.68 from one week to the next, against r = 0.84 from one month to the next for hair.⁸ In repeated measurements months apart, a stable personal component explained 59 to 82 per cent of the variation in hair cortisol.⁹ Hair also has no timing rules, since it does not vary with the time of day it is cut, and there is no collection to miss or spill.¹⁰

Do hair and urine results agree?

Less closely than you might expect, and it is worth being clear about this. In healthy adults, the studies so far have found weak to moderate agreement:

• In 17 adults, hair correlated with the average of four weekly 24-hour urine collections at r = 0.30, which was not statistically significant. The same hair samples correlated with 30 days of saliva at r = 0.61.⁸
• In 10 healthy adults who collected 24-hour urine on 63 consecutive days, hair and average urine cortisol correlated with a Kendall's tau of 0.42. The authors described this as low to moderate, and it was not statistically significant.¹¹
• In an early study, hair correlated with a single 24-hour urine collection at r = 0.33.¹²

Agreement is stronger where cortisol differs a lot between people. In patients with Cushing's syndrome, hair cortisol correlated with UFC at r = 0.69 in one study, and at r = 0.5 in another after adjusting urine for body size.¹³¹⁴

Several things may explain the gap in healthy people. The studies were small. The two tests do not cover quite the same weeks: because new hair takes time to grow out and scissors leave a stub, the centimetre nearest the scalp reflects hair formed roughly 1.3 to 2.2 months before the cut.¹⁵ Urine results depend on salt intake, urine volume and kidney function,⁷² while hair is affected by things urine is not, such as hair dye and sunlight.¹⁶¹⁷ And when everyone in a study has normal cortisol, the differences between people are small, which makes any link harder to detect. There is no perfect yardstick for cortisol over months, so neither test can be judged against a true answer. Our guide to whether hair cortisol testing is accurate covers the full evidence.

What a 24-hour urine test is for

UFC is one of the first-line tests for Cushing's syndrome, alongside late-night saliva and the dexamethasone suppression test, with accuracy similar to late-night saliva.²⁷ It is also used to monitor medical treatment for Cushing's disease.⁷ It is not how adrenal insufficiency is diagnosed: that relies on blood tests, with the short Synacthen test as the standard.¹⁸ A urine collection for Cushing's should be arranged and read by a doctor, usually as two or more collections alongside other tests. See testing for Cushing's syndrome.

One known blind spot is cyclic Cushing's syndrome, in which episodes of cortisol excess come and go. UFC can be normal if the urine is collected while the disease is inactive.²

What hair adds

A 24-hour urine collection and a late-night saliva sample each reflect at most one day.¹⁹ Hair covers months, and longer hair can be cut into segments to build a timeline. In people with confirmed and suspected cyclic Cushing's syndrome, hair timelines matched the clinical course.¹⁹ In a larger study, a single hair sample identified Cushing's syndrome with 93 per cent sensitivity and 90 per cent specificity, which the authors judged similar to the standard first-line tests, not better.¹³ Hair is not in the diagnostic guidelines, so it does not replace urine testing.⁷

Where hair helps is the everyday question of whether your cortisol has been running high or low over recent months: one cut, at any time of day, with a result that is consistent from one test to the next.⁸ See segmental hair analysis for how timelines work.

Getting each test

In the UK, a GP can arrange a urine cortisol test if your symptoms call for one.²⁰ None of the major UK providers we list sells a 24-hour urine test direct. In the US, it can be bought direct, with the collection handled through a patient service centre; the test listed below cost $55.95 as of October 2026.³

Hair tests are sold as home kits: a small cut from the back of the head, posted to a laboratory, with no container to carry around for a day. For the other comparisons, see hair vs saliva and hair vs blood.