A New Serum-Based Assay for Fat Cell-Binding Immunoglobulins: Application to the Detection of the Thyrotropin Receptor Antibodies of Graves' Disease*

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Abstract
In an accompanying report, we describe a new test for detecting and quantitating those immunoglobulins G (IgG) related to the presence of hyperthyroidism in Graves' disease. In this procedure, an immunoprecipitate formed between the test IgG and antiserum against the Fc portion of the human IgG is incubated with 125I-labeled solubilized guinea pig fat cell membranes (SFCM*). The proportion of added 125Ibound to the immunoprecipitate is a measure of fat cell-binding IgG (FBI) in the test preparation.In this report we describe an improvement of the basic technique that permitted its use with serum. Here, the test specimen of serum was allowed to interact with anti-Fc IgG coupled to beads of Sepharose-4B. SFCM* were then added, and the test proceeded as in the IgG-based procedure. Serum FBI values were decreased in a dose-dependent manner by bovine (b) TSH and, with lesser potency, by other glycoprotein hormones (bLH, bFSH, and hCG). Further, in experiments with sera and their corresponding IgG fractions from patients in the various groups studied, both individual serum FBI values and the extent to which they were decreased by theaddition of bTSH were closely correlated with the TSHbinding inhibitory (TBI) activity of the corresponding IgG fractions. These findings indicate that FBI values in the serumbased test, as in the IgG-based test, reflect mainly the concentration of IgG that bind to the TSH receptor in SFCM*. Two entirely separate evaluations of the serum-based FBI test were carried out. In the first, in sera from 21 patients with Graves' hyperthyroidism, FBI values (mean ± SD, 1.6 ± 0.6%) were completely separated from those in normal sera (−0.6 ± 0.3%; n = 20), TBI-negative sera from patients with Hashimoto's disease (−0.3 ± 0.3%; n = 21), and sera from patients with collagen-vascular disease (0 ± 0.3%; n = 16). Positive results were also obtained in sera from 2 patients with TBI-positive Hashimoto's disease and 2 with diabetes mellitus associated with anti-insulin receptor antibodies (type B diabetes mellitus). In the latter, however, abnormal FBI values were not decreased by bTSH, but were decreased by insulin, which, conversely, had no effect on the elevated FBI values found in Graves' hyperthyroidism. In the second evaluation, FBI values were measured in 34 sera from normal subjects and 38 sera from patients with hyperthyroidism due to Graves' disease; 14 samples in the former group and16 in the latter group were studied without knowledge of the diagnosis. In this evaluation, excellent, but not complete, discrimination between the 2 groups was obtained; FBI values exceeded the normal mean ± 2 SD in 31 of 38 (82%) of the Graves' disease samples. The ease, rapidity, and apparent accuracy of the FBI test, particularly as applied to serum, suggest that the test is adaptable for general clinical use.

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