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

Nina van Beek¹, Enik Bodó¹, Arno Kromminga, Erzsébet Gáspár, Katja Meyer, Michal A. Zmijewski, Andrzej Slominski, Björn E. Wenzel and Ralf Paus

Department of Dermatology (N.v.B., E.B., E.G., K.M., R.P.) and Cell and Immunobiological Laboratory (B.E.W.), Department of Medicine I, University of Lübeck, D-23538 Lübeck, Germany; Institute for Immunology, Clinical Pathology, and Molecular Medicine (A.K.), D-22339 Hamburg, Germany; Department of Pathology and Laboratory Medicine and Center for Cancer Research (M.A.Z., A.S.), University of Tennessee, Memphis, Tennessee 38163; Agricultural and Molecular Research Institute (E.B.), College of Nyíregyháza, H-4400 Nyíregyháza, Hungary; and School of Translational Medicine (R.P.), University of Manchester, Manchester M13 9PL, United Kingdom

Address all correspondence and requests for reprints to: Ralf Paus, M.D., Department of Dermatology, University Hospital Schleswig-Holstein, University of Lübeck, Ratzeburger Allee 160, D-23538 Lübeck, Germany. E-mail: ralf.paus{at}uk-sh.de.

Context: Both insufficient and excess levels of thyroid hormones (T₃ and T₄) can result in altered hair/skin structure and function (e.g. effluvium). However, it is still unclear whether T₃ and T₄ exert any direct effects on human hair follicles (HFs), and if so, how exactly human HFs respond to T₃/T₄ stimulation.

Objective: Our objective was to asses the impact of T₃/T₄ on human HF in vitro.

Methods: Human anagen HFs were isolated from skin obtained from females undergoing facelift surgery. HFs from euthyroid females between 40 and 69 yr (average, 56 yr) were cultured and treated with T₃/T₄.

Results: Studying microdissected, organ-cultured normal human scalp HFs, we show here that T₄ up-regulates the proliferation of hair matrix keratinocytes, whereas their apoptosis is down-regulated by T₃ and T₄. T₄ also prolongs the duration of the hair growth phase (anagen) in vitro, possibly due to the down-regulation of TGF-β2, the key anagen-inhibitory growth factor. Because we show here that human HFs transcribe deiodinase genes (D2 and D3), they may be capable of converting T₄ to T₃. Intrafollicular immunoreactivity for the recognized thyroid hormone-responsive keratins cytokeratin (CK) 6 and CK14 is significantly modulated by T₃ and T₄ (CK6 is enhanced, CK14 down-regulated). Both T₃ and T₄ also significantly stimulate intrafollicular melanin synthesis.

Conclusions: Thus, we present the first evidence that human HFs are direct targets of thyroid hormones and demonstrate that T₃ and/or T₄ modulate multiple hair biology parameters, ranging from HF cycling to pigmentation.