Heterogeneity of autoantibodies against thyroid peroxidase in autoimmune thyroid disease: evidence against antibodies directly inhibiting peroxidase activity as regulatory factors in thyroid hormone metabolism.

The close relationship between thyroid microsomal antigen and thyroid peroxidase (TPO) is now well established. The present study evaluates the significance of autoantibodies against TPO (anti-TPO-Abs) in the various forms and stages of autoimmune thyroid disease with respect to a possible heterogeneous nature and particularly to their influence on TPO activity. When measured by a RIA using purified human TPO, anti-TPO-Abs were highly correlated with microsomal antibodies determined by enyzme-linked immunosorbant assay (r = 0.96; P less than 0.0001) and with the results of a TPO immunoprecipitation assay using crude microsomal preparations (r = 0.76; P less than 0.001). Relating the results of these assays to the reactivities of patients' sera with thyroid microsomes in immunoblot under nonreducing and reducing conditions, discordant results could be observed in a few cases. Further analysis of these data indicate a heterogeneous nature of anti-TPO-Abs, which react with at least two antigenic domains of the TPO molecule. The comparative analysis of patients with hyperthyroid Graves' disease, patients with Graves' disease in clinical remission, and patients with hypothyroid Hashimoto's thyroiditis revealed no significant differences in the antibody spectrum. When evaluating the direct influence of anti-TPO-Abs on the activity of TPO under a rigorous methodological approach, we found no significant inhibition of the enzymatic activity by any of the sera investigated from patients with autoimmune thyroid disease compared to that in sera from normal controls. In conclusion, the data indicate a heterogeneous nature of anti-TPO-Abs. The spectrum of antigenic epitopes recognized by anti-TPO-Abs seems not to be significantly different in the various forms and stages of autoimmune thyroid disease. The lack of an inhibitory effect of autoantibodies on TPO activity argues against direct binding of autoantibodies to the enzymatic sites of TPO and indicates that they are not important factors in producing thyroid dysfunction in autoimmune thyroid disease.