A large number of socially significant diseases is accompanied with oxidative stress and carry with tissue damage. Free radicals play a crucial role in the development of these diseases. Similar processes occur under the influence of ionizing radiation and bacterial infections. Recently, was indicated the significant role of oxidative stress in the development of autoimmune thyroiditis. It is assumed that the synthesis of thyroid hormones depends on the concentration of H2O2, which, due to its high toxicity, must be in strict accordance with the activity of antioxidant systems. Many biochemically negative processes occur on the apical membrane of the thyrocyte, which allows limiting the effect of free radicals and avoid cell destruction. However, in pathological conditions, enzymatic systems are disturbed and their components become abnormally activated in the cytoplasm, and it is leads to functional and morphological disorders. A deeper understanding of oxidative stress and its role in the development of autoimmune thyroiditis can contribute to the identification of new methods for its assessment, the expansion of therapeutic ranges for this disease. This review discusses oxidative stress, which is the accumulation of active damaging agents (free radicals, prooxidants, reactive oxygen species) that initiate cell damage and lead to the development of various pathological conditions.