Thyroid Hormones Directly Alter Human Hair Follicle Functions: Anagen Prolongation and Stimulation of Both Hair Matrix Keratinocyte Proliferation and Hair Pigmentation

In 2008, researchers discovered that thyroid hormones, particularly T3 and T4, have a direct impact on human hair follicle functions. The study, which used anagen hair follicles from euthyroid females aged 40 to 69, demonstrated that T4 increases the proliferation of hair matrix keratinocytes and decreases their apoptosis, while both T3 and T4 reduce apoptosis and prolong the anagen phase of hair growth. This anagen prolongation is likely due to the suppression of TGF-B2, an inhibitor of hair growth. Furthermore, T3 and T4 were found to significantly boost melanin synthesis within the hair follicle, indicating a role in hair pigmentation. The study also showed that hair follicles can potentially convert T4 to T3, as they express deiodinase genes. These findings suggest that thyroid hormones are key regulators of hair follicle cycling, pigmentation, and keratin expression, which could have implications for treating hair growth disorders.