Hair as a retrospective calendar of cortisol production – increased cortisol incorporation into hair in the third trimester of pregnancy

Background: Hair has long been used as a biological specimen for the detection of environmental agents, drugs, or toxins. Recent evidence suggests that also hormones are incorporated and trapped inside the growing hair. Therefore, hair segmental analysis could provide a retrospective calendar of cortisol production for the individual. Methods: In this first proof-of-concept study in humans, we analyzed cortisol in hair donated by mothers with a neonate child (n-Mothers; N=103), mothers with toddlers 3–9 months of age (t-Mothers; N=19), and control women (N=20). We cut hair strands into at least three 3-cm segments, which would represent hair grown over the past nine months. Since in the third trimester of pregnancy there is a well-documented increase in cortisol production, we expected to see elevated levels of cortisol in the most proximal hair segment of n-Mothers compared with the control women. Likewise, we expected to see elevated levels in the second, third, or fourth segment of mothers of 3-month olds, 6-months olds, and 9-months olds, respectively. Results: There was a monotonic decline in cortisol concentration from the scalp-near segment to the most distal hair segment (p<0.0001). Cortisol levels in the first 3-cm hair segment of n-Mothers were two-fold higher than in controls (p=0.0001). No differences in cortisol content were apparent for the second or third 3-cm segements (p>0.2). When hair from t-Mothers was analyzed, the hair segment representing the 3^rd trimester of pregnancy contained the same amount of cortisol as the hair grown more recently. Age of the women, hair curvature, hair colour, and frequency of hair washes per week were unrelated to cortisol levels. Conclusions: We conclude that cortisol measured in human hair can be a valid reflection of increased cortisol production for a period of up to six months. Due to a decline of cortisol levels in human adult hair, a retrospective calendar of cortisol exposure may be limited to the past six months.