Hair and nail cortisol levels are associated and affected by glucocorticoid use

Hair and nail cortisol is increasingly studied as a physiologic proxy for chronic stress response. Glucocorticoid use is an expected confounder for cortisol measurement, yet there remains little evidence of whether external cortisol use should be subject to exclusion in study subjects. In a group of 209 youth (15-22 year-olds), we analyzed hair and fingernail cortisol concentrations. We assessed topical, nasal, oral, and injectable glucocorticoid use via a questionnaire. Extensively validated methods were used for hair and nail cortisol extraction and measurements. The median value of hair cortisol was 10.2 pg/mg (n=200), and the median value of nail cortisol was 7.06 pg/mg (n=203). Topical glucocorticoid use significantly increased hair and nail cortisol concentrations (p<0.005). Hair and nail cortisol concentrations were positively associated (p<0.0001, n=194). Spearman correlation coefficients demonstrated that the positive correlation between hair and nail cortisol values was higher in participants who used external glucocorticoids. Topical glucocorticoids moderated the association between hair and nail cortisol values (p=0.006). Based on these findings, we recommend that the assessment of topical glucocorticoid use must be performed when collecting hair/nail samples and that subjects reporting glucocorticoid use should be excluded from all future hair and nail cortisol studies; also, all outliers must be excluded to account for glucocorticoid medication underreporting and yet-unknown confounders.