Hashimoto's thyroiditis (HT) is a chronic autoimmune thyroid disease caused by an interaction between genetic factors and environmental conditions, both of which are not yet completely understood. The significant association between vitamin D deficiency and HT has been investigated regarding the immune role of this hormone. In HT, an immunologic reaction is triggered when thyrocytes express major histocompatibility complex (MHC) class II surface HLA-DR antigens, a process induced by the production from T helper (Th)1 type lymphocytes, of inflammatory cytokines (especially IFN-γ), which may be inhibited by 1,25[OH]2D. Genetic polymorphism of vitamin D receptor (VDR), binding protein (DBP) and of 1α-hydroxylase (CYP1α) may also predispose to the development of HT. Considering current evidence, presented in this review, screening for vitamin D deficiency and careful vitamin D supplementation, when required, may be recommended for patients with HT. Further research is needed in patients with HT in order to investigate the mechanisms by which vitamin D affects autoimmunity and also to evaluate the cost-effectiveness of vitamin D supplementation and to suggest the possible optimal dose treatment.