TLDR Hair cortisol can indicate long-term stress levels, but hair growth rate differences may affect accuracy.
This study validated the incorporation of cortisol into hair as a measure of hypothalamic-pituitary-adrenal (HPA) axis activity by using tritiated [3H]-cortisol in rhesus monkeys. Seven adult male monkeys were injected with [3H]-cortisol, and hair samples were collected at 14 and 28 days post-injection. The study found that [3H]-cortisol was incorporated into the hair as [3H]-cortisol, [3H]-cortisone, and other metabolites, supporting the hypothesis that hair cortisol reflects circulating cortisol levels. The results indicated that cortisol diffuses from the bloodstream into hair follicles, validating hair cortisol as a non-invasive biomarker for long-term HPA axis activity. However, individual hair growth rates varied, suggesting that segmenting hair to measure cortisol over specific time periods may not be reliable.
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