Assays for Thyroid-Stimulating Hormone Receptor Antibodies Employing Different Ligands and Ligand Partners May Have Similar Sensitivity and Specificity but Are Not Interchangeable

Background: The best biochemical marker of Graves' disease (GD) is the presence in serum of autoantibodies to the thyroid-stimulating hormone receptor (hTSHR-Ab). The aim of this study was to evaluate the performances of two sensitive hTSHR-Ab assays with a specific focus on the clinical importance of differences in results. Both assays are competitive in nature but employ quite different types of ligands. In the M22-pTSHR'' assay, hTSHR-Ab competes with a labeled monoclonal antibody (M22*) against the thyrotropin (TSH)-receptor for binding to porcine TSH receptors. In the bTSH-rhTSHR'' assay, hTSHR-Ab competes with labeled bovine TSH for binding to recombinant human TSH receptors. Methods: bTSH-rhTSHR and M22-pTSHR were measured in patients from a population study: 106 had new hyperthyroidism due to GD, 93 had multinodular toxic goiter, 100 had new primary autoimmune hypothyroidism, and 100 were healthy controls. Results: Receiver operating characteristic curves indicated a high sensitivity and specificity of both assays (area under curve, bTSH-rhTSHR: 0.977 [confidence interval: 0.954-1.00]; M22-pTSHR: 0.979 [confidence interval: 0.957-1.00]). The two assays identified nearly the same patients who were hTSHR-Ab positive, though large differences in hTSHR-Ab values were obtained in a number of individual patients (ratio bTSH-rhTSHR/M22-pTSHR, range: 0.33-6.5 in patients positive with both assays). Values were in average 2.5 times higher with the bTSH-rhTSHR assay compared with the M22-pTSHR assay, corresponding to the difference in recommended clinical cut-off values (1.0 IU/L and 0.4 u/L). The bTSH-rhTSHR assay had a considerably lower intraassay coefficients of variation of 3.8%; for M22-pTSHR, it was 9.5% (p < 0.01). Conclusions: Both assays had a high sensitivity and specificity for diagnosing GD. hTSHR-Ab values were in average 2.5 times higher with the bTSH-rhTSHR assay compared with the M22-pTSHR assay. In individual patients, the ratio between results obtained using the two assays varied widely. Thus, results obtained using one assay cannot be quantitatively transformed to values obtained using the other assay. bTSH-rhTSHR had a considerably lower intraassay coefficients of variation and it may be better suited for longitudinal studies of hTSHR-Ab.