Hair Cortisol Testing: What a Strand of Hair Can (and Can't) Reveal About Long-Term Stress

Most cortisol tests capture a single moment. Blood and saliva levels rise and fall through the day, peaking shortly after waking and dropping toward bedtime, and they shift with meals, exercise, and even the stress of having a test. Over the past two decades, researchers have turned to a different sample to get a longer view: a small lock of hair cut close to the scalp. Because cortisol from the bloodstream is gradually incorporated into growing hair, a hair sample can serve as a rough record of hormone exposure over weeks to months.

The basic logic is simple. Scalp hair grows by roughly one centimeter per month on average, so a three-centimeter segment nearest the scalp is often treated as an approximate summary of the past three months. Labs wash the hair, grind it, extract the cortisol, and measure it. A widely cited 2012 review by Elizabeth Russell and colleagues in Psychoneuroendocrinology described this method as a promising biological marker of chronic stress while also listing many unanswered questions, including how exactly cortisol enters hair and how much washing and cosmetic treatment wash it back out.

The largest summary of the evidence so far is a 2017 meta-analysis led by Tobias Stalder, also in Psychoneuroendocrinology, which pooled 66 studies with more than 10,000 participants. It found that people facing ongoing, chronic stressors, such as long-term unemployment, caregiving, or persistent pain, tended to have higher hair cortisol on average. Interestingly, hair cortisol showed little relationship with how stressed people said they felt on questionnaires. The analysis also found that factors unrelated to stress, including body weight, use of hormonal contraceptives, hair washing frequency, and hair dyeing or other treatments, were associated with measurable differences in results.

That last point is why hair cortisol works better for research than for personal diagnosis. In a study comparing hundreds of people, individual quirks tend to average out, and real group differences can appear. For one person, a single reading is harder to interpret, because there is no widely accepted normal range for the general public, results vary between laboratories, and hair care habits can shift values on their own. A 2019 review by Greff and colleagues in Clinical Biochemistry noted that methods are still being standardized and that clinical uses remain limited. Endocrinologists have explored hair cortisol as a possible supporting tool for conditions like Cushing's syndrome, but diagnosis still relies on established tests such as late-night saliva, 24-hour urine, and suppression tests.

For skin, the connection is even more indirect. Some research has looked at hair cortisol in people with conditions like psoriasis or eczema, but these studies are small and findings are mixed. A hair test cannot tell you whether a particular breakout, rash, or "puffy face" is caused by cortisol, and there is no evidence that lowering a hair cortisol number improves skin.

So what is proven and what is trend? Reasonably well supported: hair cortisol reflects longer-term cortisol exposure and, across large groups, tends to be higher in people under chronic stress. Still uncertain: how to interpret one person's result, given the lack of standard reference ranges and the influence of hair treatments. Trend and marketing: direct-to-consumer kits that promise to reveal your "true stress level" or diagnose "adrenal fatigue" from a hair sample. If you are worried about your cortisol, a doctor can order validated tests and interpret them in context.