A distribution of immunoglobulin G (IgG) subclass of anti-thyroid peroxidase (TPO) autoantibodies was studied to know whether anti-TPO autoantibodies are closely implicated in the pathogenesis of human autoimmune thyroid diseases. As a result of analyzing 14 patients' sera, 7 with Graves' disease and 7 with Hashimoto's thyroiditis, anti-TPO autoantibodies were found to consist of mainly IgG₁ subclass. Percentages of both IgG₁ and IgG₂ subclasses in IgG class of autoantibodies corresponded to those in the normal serum composition, whereas IgG₃ subclass was scarcely contained in anti-TPO autoantibodies and IgG₄ subclass markedly increased. It was thought that anti-TPO autoantibodies had a capability to lyse thyroid follicular cells by the mechanism of antibody-dependent complement-mediated cytolysis, because IgG₁ and IgG₂ subclasses of antibodies can fix complement and TPO locates in apical membrane surface of thyroid follicular cells.
Comparing Graves' disease with Hashimoto's thyroiditis, mean percentages of both IgG₁ and IgG₂ subclasses of 2 groups were statistically different. Namely, sera of patients with Graves' disease had higher and lower mean percentages of IgG₁ and IgG₂ subclasses of autoantibodies, respectively, than those with Hashimoto's thyroiditis, though no plausible explanation for these differences can be offered at the present time.