Background: Low vitamin D status is associated with autoimmune thyroid disease. Oral vitamin D supplementation was found to reduce titers of thyroid antibodies in levothyroxine-treated women with postpartum thyroiditis and low vitamin D status. Methods: The study included 34 women with Hashimoto's thyroiditis and normal vitamin D status (serum 25-hydroxyvitamin D levels above 30 ng/mL) who had been treated for at least 6 months with levothyroxine. On the basis of patient preference, women were divided into 2 groups, receiving (n=18) or not receiving (n=16) oral vitamin D preparations (2000 IU daily). Serum levels of thyrotropin, free thyroxine, free triiodothyronine and 25-hydroxyvitamin D, as well as titers of thyroid peroxidase and thyroglobulin antibodies were measured at the beginning of the study and 6 months later. Results: There were no significant differences in baseline values between both study groups. 25-hydroxyvitamin D levels inversely correlated with titers of thyroid antibodies. No changes in hypothalamic-pituitary-thyroid axis activity and thyroid antibody titers were observed in vitamin-naïve patients. Vitamin D increased serum levels of 25-hydroxyvitamin D, as well as reduced titers of thyroid antibodies. This effect was more pronounced for thyroid peroxidase than for thyroglobulin antibodies and correlated with their baseline titers. Conclusions: Vitamin D preparations may reduce thyroid autoimmunity in levothyroxine-treated women with Hashimoto's thyroiditis and normal vitamin D status.
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