Hair follicles are sensitive to hormones, including vitamin D, which plays an important role in calcium metabolism, immune system regulation as well as cell growth and differentiation. The active form of vitamin D, 1,25 dihydroxyvitamin D3 [1,25(OH)2D3], performs its actions by binding to specific vitamin D receptors (VDRs) in the nuclei of target cells, which act as ligand-inducible transcription factors, regulating vitamin D-responsive genes. Vitamin D receptors are expressed in the outer root sheath (ORS), hair follicle bulb, and sebaceous glands in the hair follicle,where they regulate skin biology, as epidermal proliferation and differentiation. In addition, studies demonstrated that VDRs are important for normal hair cycling, especially anagen initiation.
Alopecia areata (AA), an inflammatory hair loss whose etiology is yet to be established, has been reported to occur with vitamin D deficiency, vitamin D-resistant rickets, and VDR mutation. Androgenetic alopecia (AGA) is a noncicatricial or potentially reversible type of patterned hair loss.
Significantly lower serum vitamin D levels were demonstrated in female patients with AGA in comparison with female controls.7 However, VDRs have not been assessed before in AGA.