Further evaluation of an immunoprecipitation assay for TSH-receptor autoantibodies in Graves' disease.

In ten patients with untreated Graves' disease, quantitative titers of TSH receptor antibodies, as measured by a recently developed immunoprecipitation assay (IPA), were correlated with results obtained in three other methods and with the severity of the hyperthyroidism, as assessed by thyroid function tests. Nine patients were positive in the IPA, six in the TBII (TSH-binding inhibitor immunoglobulins), six in the TSI (thyroid-stimulating immunoglobulins), and seven in the TGI (thyroid-growth stimulating immunoglobulins) assay. Three patients were positive in all four assays. No correlation was found between the IPA values and the results obtained in the TBII, TSI, or TGI assays. There was a modest correlation between the TBII and TSI assays (r = .74, P less than 0.02). There was a modest but significant correlation between the IPA titers of TSH receptor antibodies and serum T3 concentration, both before treatment (r = .63, P less than 0.05) and during treatment (n = 5; r = .84, P less than 0.05). No correlation between the severity of the hyperthyroidism and TBII, TSI, or TGI assays was observed. Finally, TGI did not correlate with goiter size as estimated by palpation. These results suggest that the IPA may be useful in monitoring the immunologic activity of TSH receptor antibodies in patients with Graves' disease. They suggest further that the IPA detects a number of different subpopulations of TSH receptor antibodies, including TSI, TBII, and perhaps TGI. This property may be particularly useful in screening for monoclonal TSH receptor antibodies.