Here we demonstrate that physiological concentrations of the thyroid hormones T3 
and T4 enhance the KERATIN 15 promoter activity and expression in epithelial 
stem cells of adult human scalp hair follicles in situ and in vitro. 
Additionally, T3 and T4 stimulate expression of the immuno-inhibitory surface 
molecule CD200. Subsequently, T3 and T4 induce apoptosis and differentiation and 
inhibit clonal growth of these progenitor cells in vitro. These data suggest 
that human hair follicle bulge-derived epithelial stem cells underlie profound, 
previously unknown hormonal regulation by thyroid hormones, and show that 
primary human keratin 15-GFP+ progenitor cells can be exploited to further 
elucidate fundamental endocrine controls of human epithelial stem cells.