Repigmentation of Gray Hair After Thyroid Hormone Treatment

In the study, the repigmentation of gray hair was observed in two patients after increased exogenous triiodothyronine (T3) due to different thyroid conditions. This led to the hypothesis that thyroid hormone could influence hair follicle homeostasis. To investigate this, both an in vivo murine model and an in vitro culture of follicular units were used. In the in vivo experiment, T3 was topically applied to mice in the telogen phase, resulting in all T3-treated mice entering the anagen phase by day 5, compared to control mice which started entering anagen on day 9 and completed by day 15. In the in vitro experiment, follicular units treated with 100 nmol/L T3 showed significant growth compared to controls. These results suggest that T3 can induce hair follicles in the telogen phase to enter the anagen phase and stimulate hair shaft growth, with follicular melanocytes possibly being the target cells for T3's actions.