VDR and psoriasis: In addition, UVB therapy increased serum 25(OH)D levels in patients with psoriasis in parallel with disease improvement.26 The exposure of the skin to sunlight is the major source of vitamin D. Moreover, the epidermis and hair follicle keratinocytes express the hydroxylases needed to produce the active hormone 1,25-Dihydroxyvitamin D [1,25(OH)2D], and vitamin D receptor has been shown on keratinocytes.