Is there a relation between major depression and hair cortisol? A systematic review and meta-analysis

Literature supports a causal role of stress in major depressive disorder (MDD). Hair cortisol concentration (HCC) has been widely used as a measure of long-term stress. Although elevated HCC has been observed in healthy people experiencing chronic stress, findings regarding individuals with mental disorders have been complicated. This review attempts to systematically present all the published research on major depression and HCC. An extensive search of databases was performed to identify articles that investigated this question. The initial search retrieved 142 studies, of which, 16 original articles were included in this review. Results were contradictory; most of the studies showed no significant HCC differences between MDD patients and controls, while others indicated either higher or lower HCC in MDD patients than controls. Higher HCC was reported in first depressive episode compared to recurrent MDD and controls; patients with comorbid MDD and anxiety disorder had higher HCC than controls. No significant HCC difference was found between patients with melancholic or atypical depression and controls. Findings concerning HCC in postpartum depression were inconsistent. A meta-analysis of the data extracted from seven studies of the sample was performed to quantify the degree of cortisol change in MDD patients vs. controls. A random effects model revealed no significant hair cortisol concentrations difference between depressed patients and healthy controls (SMD: -0.02, 95% CI: -0.36 to 0.32). Significant heterogeneity was identified across included studies (P = 0.002, I² = 71%). The disagreement among studies' results indicates that there is room for improvement in this research field. Confounding factors independent of depression should be taken into consideration.