Thyrotropin-releasing hormone (TRH) is the most proximal component of the hypothalamicpituitary-thyroid axis that regulates thyroid hormone synthesis. Since transcripts for members of this axis were detected in cultured normal human skin cells and since human hair follicles (HFs) respond to stimulation with thyrotropin, we now have studied whether human HF functions are also modulated by TRH. Here we report that the epithelium of normal human scalp HFs expresses not only TRH receptors (TRH-R) but also TRH itself at the gene and protein level. Stimulation of microdissected, organ-cultured HFs with TRH promotes hair-shaft elongation, prolongs the hair cycle growth phase (anagen), and antagonizes its termination by TGF-β2. It also increases proliferation and inhibits apoptosis of hair matrix keratinocytes. These TRH effects may be mediated in part by reducing the ATM/Atr-dependent phosphorylation of p53. By microarray analysis, several differentially up- or down-regulated TRH-target genes were detected (e.g., selected keratins). Thus, human scalp HFs are both a source and a target of TRH, which operates as a potent hair-growth stimulator. Human HFs provide an excellent discovery tool for identifying and dissecting nonclassical functions of TRH and TRH-mediated signaling in situ, which emerge as novel players in human epithelial biology. © FASEB.