Background: Phototherapy and activated forms of vitamin D help clear psoriasis.

Objective: The influence of UVB phototherapy and oral calcitriol (1,25-dihydroxyvitamin D₃) on vitamin D photosynthesis was assessed in 16 patients.

Methods: Patients were randomly selected to receive orally either placebo or calcitriol (0.5 to 2 μg daily) for the duration of the 8-week study; all patients received approximately 21 UVB treatments. Before and after treatment, serum levels of 25-hydroxyvitamin D and calcitriol were measured by high-pressure liquid chromatography.

Results: Although calcitriol had no additive effect on phototherapy as a treatment modality, a significant increase in serum 25-hydroxyvitamin D levels occurred in both groups; in three patients extraordinarily high levels developed (>120 ng/ml). Oral calcitriol significantly increased calcitriol serum levels. Increased serum calcitriol did not inhibit cutaneous synthesis of vitamin D or its hepatic conversion to serum 25-hydroxyvitamin D.

Conclusion: UVB induces high levels of vitamin D photosynthesis. Because oral or topical calcitriol alone helps clear psoriasis, studies to explore the possible influence of UVB phototherapy on its production should be considered. If UVB phototherapy induces cutaneous calcitriol synthesis this could explain the lack of added benefit to treatment when oral calcitriol is administered with phototherapy.