Vitamin D administered together with selenomethionine has been reported to markedly reduce thyroid antibody titres in patients with autoimmune thyroiditis. Hyperprolactinaemia exerts a complex pro-inflammatory effect. This study was aimed at investigating whether prolactin excess determines the effect of vitamin D/selenomethionine combination therapy on thyroid autoimmunity.
The study included two age-, body mass index-, hormone- and thyroid antibody-matched groups of young euthyroid women with Hashimoto’s thyroiditis: 19 women with mild-to-moderate hyperprolactinaemia and 19 individuals with prolactin levels within the reference range. All participants were then treated with vitamin D (4000 IU daily) and selenomethionine (200 µg daily). Serum titres of thyroid peroxidase and thyroglobulin (TgAb) antibodies, serum levels of thyrotropin, free thyroxine, free triiodothyronine, prolactin and 25-hydroxyvitamin D, as well as the calculated parameters of thyroid homeostasis, were determined at baseline and 6 months later.
All women completed the study. With the exception of prolactin and 25-hydroxyvitamin D levels, there were no differences between the study groups in the investigated parameters. In both study groups, vitamin D/selenomethionine combination therapy reduced thyroid peroxidase and TgAb antibody titres, decreased the free thyroxine:free triiodothyronine ratio and increased 25-hydroxyvitamin D levels and SPINA-GD. The decrease in antibody titres, as well as the improvement in vitamin D status, was more pronounced in subjects with prolactin levels within the reference range than in subjects with hyperprolactinaemia and was inversely correlated with prolactin levels. Moreover, only in normoprolactinaemic women, the treatment reduced thyrotropin levels and increased SPINA-GT.
The obtained results suggest that hyperprolactinaemia attenuates the impact of vitamin D/selenomethionine combination on thyroid autoimmunity.

© 2020 John Wiley & Sons Ltd.