IL17A and autoimmune thyroid disease: Patients with autoimmune diseases were shown to have significantly lower 25 (OH) D3 levels.8,9 A significant relationship was detected between vitamin D gene polymorphism and autoimmune thyroid diseases.10 Vitamin D plays an important role in regulation of T helper cell type 1 (Th1), Th2 and Th 17cells and IFN-gamma, IL-4, IL-17 secretion with its immune-modulator effect.