Interference of Serum Thyroglobulin in the Radioassay for Serum Antithyroglobulin Antibodies

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Abstract
Parallel measurements of serum antithyroglobulin (anti-Tg) antibody by competitive binding radioassay and tanned red cell (TRC) agglutination were performed in subjects with and without thyroid disorders. Radioassays were carried out using a partially purified preparation of anti-Tg antibody obtained by affinity chromatography using human thyroglobulin (Tg) coupled to Sepharose 4B. Two-thirds (67.2%) of control subjects had undetectable antibody levels (P < 0.001) between the two methods, but clearly elevated values by radioassay were found in several TRC-negative samples. The most important discrepancies between the two methods were found in sera of patients with metastatic differentiated thyroid carcinoma; discrepancies of intermediate magnitude were frequently found in Graves' disease, while little discrepancy was observed in patients with a toxic adenoma. Experiments performed on representative sera indicated that, unlike circulating anti-Tg antibody, the substance which caused a positive radioassay response without producing TRC agglutination was not associated with the IgG fraction and could not be removed by immunoadsorption with Tg-Sepharose 4B. Furthermore, addition of Tg to the radioassay system produced a dose-dependent inhibition of tracer binding. A significant inhibition of tracer binding was observed with as little as 4.8 ng of Tg (corresponding to a serum concentration of 48 ng/ml). Moreover, determination of serum Tg by radioimmunoassay (RIA) on 180 TRC-negative sera showed that the concentration of Tg increased progressively with the increase in the apparent anti-Tg antibody level as assessed by radioassay. Evidence has been provided that increased serum Tg levels may produce false positive results in measurements of anti-Tg antibody by competitive binding radioassay. Caution should be exercised in the interpretation of the data obtained by this method.