A New Solid Phase Immunoradiometric Assay for Antithyroid Microsomal Antibody*

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Abstract
A new sensitive, quantitative, and specific immunoradiometric assay (IRMA) for antithyroid microsomal (anti-M) antibody has been developed. Samples to be tested are incubated within wells of polyvinyl microtiter plates coated with solubilized thyroid microsomal antigen. After removal of unbound material, anti-M antibody is detected by adding purified [125I]antihuman immunoglobulin G (IgG) antibody. Using 1.0 µ1 serum, anti-M antibody was found by IRMA in all of the patients with Hashimoto's thyroiditis or idiopathic myxedema (n = 19), in 86÷ of those with Graves' disease (n = 42), in 10.9÷ of subjects with other nonautoimmune thyroid disorders (n = 37), and in 8.4÷ of normal controls (n = 71). A good correlation was found with the results obtained in anti-M antibody tests by passive hemagglutination. Using larger volumes of serum (up to 100 µl), anti-M antibody detectable by IRMA was found in some patients with Graves' disease and negative passive hemagglutination tests. Quantitative measurements of anti-M antibody by IRMA could be performed using a standard IgG preparation containing high levels of anti-M antibody. The minimal detectable amount ranged between 1–2 ng IgG, corresponding to a sensitivity 15–30 times greater than that of the competitive binding radioassay. We suggest that the present IRMA may be proposed as a general technique for the detection of different organ-specific autoantibodies. (J Clin Endocrinol Metab56: 467, 1983)