Hair cortisol concentrations in mental disorders: A systematic review

•Hair cortisol was elevated in drug-naïve first-episode patients with psychosis, compared to matched controls; the differences seem to associate with the severity of psychopathology.•Results concerning HCC in people with depression or anxiety disorders are rather inconsistent.•PTSD patients show significantly lower HCC than trauma-exposed controls without PTSD.•The timing of cortisol sampling seems to be vital for explaining the differences of the indicated results.•Associations of hair cortisol concentrations with self-reported stress measures are inconclusive. Research on HPA axis dysregulation has been associated with vulnerability to, or perseverence of, several mental disorders. Hoever, measurements of cortisol levels in blood, saliva and/or urine have yielded variable results. Nevertheless, cortisol analysis in scalp hair appears to be a consistent tool for measurement of long-term exposure to stress. This article provides a systematic review of studies exploring hair cortisol concentrations in patients with mental disorders in comparison with healthy controls. This review was conducted according to PRISMA guidelines. The electronic databases of PubMed/Medline, Web of Science and Scopus were searched for relevant articles, using a specific syntax. A total of 582 articles were identified, of which 22 were finally included. Patients with depression show a general trend for higher hair cortisol concentrations (HCC) than controls, whereas patients with PTSD tend to demonstrate lower HCC. Very little is known about other mental disorders, including suicidality and drug abuse. The divergence of samples included and the timing of cortisol sampling, seem to play a key role in the discrepancies of the results. Correlations of HCC with self-reported measures of stress were found, at best, inconclusive. Further research should attempt to describe specific cortisol profiles for each psychiatric disorder and HCC could contribute in evaluating therapy outcomes and predicting relapses. Obtaining information on HCC in different stages of psychiatric disorders in association with pertinent clinical variables, might help in forging a neuroendocrine model for clinical staging of mental disorders.