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

Heterogeneity of TSH receptor-binding antibodies in Hashimoto's thyroiditis and Graves' disease

The possible heterogeneity of TSH receptor antibodies in Graves' disease (GD) and Hashimoto's thyroiditis (HT) with respect to the binding site on the receptor and corresponding biological effect was studied. Employing an immunoprecipitation assay (IPA), the sera of 80% of the patients with GD (24 out of 30) and 76% of the patients with HT (16 out of 21) contained TSH receptor-binding antibodies, compared to none of the sera from 17 normal volunteers and 8 patients with nontoxic multinodular goiter. TSH inhibited immunoprecipitation by GD and HT sera. In HT sera (n = 9), but not in GD sera (n = 5), heterogeneity of the TSH-induced inhibition was observed. Four HT sera showed complete inhibition of immunoprecipitation at a saturating concentration (19.8 nM) of TSH. Five HT sera, like the 5 GD sera, showed partial inhibition of immunoprecipitation by 19.8 nM TSH. Thyroid stimulating immunoglobulins (TSI) were found in four of the five GD sera and in only one of the nine HT sera. The results suggest that different subpopulations of TSH receptor antibodies, characterized by other receptor binding sites or different affinities, are associated with autoimmune thyroid disease.

The thyroid stimulating hormone receptor in human disease

The initial step in the action of thyrotropin (TSH) is its binding to the TSH receptor. TSH receptor antibodies are detected in up to 90% of patients with Graves' disease. Serial measurements of TSH receptor antibodies in patients with Graves' hyperthyroidism are helpful in predicting relapse. The TSH receptor was purified using affinity chromatography on wheat germ lectin agarose and TSH-agarose. Using an immunoblotting technique to characterize the TSH receptor, it was found to be an oligomeric glycoprotein consisting of three noncovalently bound subunits of Mr approximately 70,000, approximately 50,000 and approximately 35,000 which on reduction yield a single subunit of Mr approximately 25,000.

Difference in species specificity of TSH receptor antibodies in Graves' disease and Hashimoto's thyroiditis

TSH receptor antibodies have been detected in the sera of patients with Graves' disease (GD) and Hashimoto's thyroiditis (HT). Since non-human thyroid tissue fractions or cells are used in the majority of assays, the species specificity of TSH receptor antibodies in GD or HT could be important. The species specificity was evaluated by means of an immunoprecipitation assay (IPA) using Triton X-100 solubilized TSH receptors prepared from human, porcine and rat thyroid as well as guinea pig fat cells (GPFC). In each assay the majority (4 or 5) of the 6 patients with GD were IPA-positive. In contrast, 9 out of 11 patients (82%) with HT had a positive human and rat IPA, while only 3 out of 11 (27% p less than 0.05) sera were positive in the porcine and GPFC assays.

CONCLUSIONS

no species specificity of TSH receptor antibodies was detected for patients with GD; a selective species specificity for human and rat TSH receptors was found for HT sera. This suggests that TSH receptor antibodies in HT either recognize different determinants on the receptor than the antibodies in GD or are of lower affinity. Furthermore, the use of porcine thyroid tissue or GPFC may lead to an underestimation of the presence and level of TSH receptor antibodies in patients with HT.