Interference of serum thyroglobulin in the radioassay for serum antithyroglobulin antibodies

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 (<20 U/ml) by this method and only 3.1% had concentrations of ≥60 U/ml. Abnormally elevated levels (≥60 U/ml) were found in the majority of the patients with Hashimoto's thyroiditis (88.9%) or idiopathic myxedema (69.3%), in half of those with untreated (54.8%) or treated (47.1%) Graves' disease, and only in a minority of those with pituitary hypothyroidism (11.1%) or non toxic goiter (9.5%). A number of patients with toxic adenoma (37.5%) or thyroid carcinoma (27.2%) also had increased antibody concentrations by radioassay. The percentage of TRC-positive sera in thyroid carcinoma (2.3%), toxic adenoma (6.3%), and in untreated (21.6%) or treated (30.8%) Graves' disease was definitely lower than that of abnormal values observed in the radioassay. This could not merely be explained by a greater sensitivity of the latter procedure, since such a discrepancy was not observed in other groups. Direct comparison of parallel tests on a total of 786 sera revealed a highly significant correlation (r = 0.66, 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. © 1977 by The Endocrine Society.