Topical vitamin D has long been a treatment for psoriasis vulgaris, but a recent review of the literature support oral vitamin D as also efficacious. The literature also suggests a relationship between low serum vitamin D levels and severity of disease, as well as oral vitamin D positively affecting psoriatic arthropathy.
J Dermatolog Treat. 2013 Aug;24(4):261-7. (PMID: 22103655)