A new solid phase immunoradiometric assay for antithyroid microsomal antibody.

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 microliter 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 microliters), 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.