Comparison of Serum Thyroid Microsomal and Thyroid Peroxidase Autoantibodies in Thyroid Diseases*

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Abstract
Recent evidence indicates that human thyroid peroxidase (TPO) has most of the characteristics of the thyroid microsomal antigen. The question of whether TPO accounts for part of all of the antigenic activity recognized by circulating anti-microsomal antigen autoantibody (anti-M Ab) remains to be determined. The availability of an anti-TPO monoclonal antibody and of a highly purifed TPO preparation allowed the development of specific and sensitive radioassays for anti-TPO autoantibody (anti-TPO Ab). In this study we compared anti-M Ab and anti-TPO Ab levesl in serum from 128 subjects, including patients with Hashimoto''s thyroiditis (n = 31), idiopathic myxedema (n = 11), hyperthyroid Graves'' disease (n = 45), miscellaneous nonautoimmune thyroid disorders (n = 9), and normal subjects (n = 32). Anti-M Ab and anti-TPO Ab were measured by radioimmunological methods employing two different assay designs: 1) competitive radioassay (CR), based on the inhibition of radioiodinated antibody binding to human thyroid microsomes coated on microtiter wells, using a) [125I]immunoglobulin G (IgG) containing a high anti-M Ab titer (for anti-M Ab determinations), or b) [125I]anti-TPO monoclonal antibody (for anti-TPO Ab); and 2) sandwich immunoradiometric assay (IRMA) using microtiter wells coated with thyroid microsomes for anti-M Ab determinations) or immunoaffinity-purified TPO (for anti-TPO Ab determinations) and [125I]anti-human IgG antibody. Anti-M Ab also was measured by passive hemagglutination. Anti-M Ab titers by PH closely correlates with anti-TPO Ab levels whether assayed by IRMA (r = 0.905; P < 0.00001) or CR (r = 0.922; P < 0.00001). Even closer correlations were found when anti-M Ab and anti-TPO Ab both were measured by the same type of radioassay procedure (IRMA, r = 0.945 and P < 0.00001; CR, r = 0.957 and P < 0.00001). No differences in the correlation between anti-M Ab and anti-TPO Ab results were found when the data in patients with different autoimmune thyroid disorder were analyzed separately. Further and more direct evidence for the identity of anti-M Ab and anti-TPO Ab was provided by the ability of purified TPO completely inhibit the binding to thyroid microsomes of radioiodinated IgG preparations containing high anti-M Ab titers. In conclusion, our results provide strong support for the concept that TPO accounts for virtually all of the antigenic determinants reacting with the autoantibodies commonly termed anti-M antibodies. This conclusion is based on correlational analyses of anti-TPO and anti-M antibody titers by multiple assay techniques, corroborated by inhibition studies.