A novel bioassay for anti‐thyrotrophin receptor autoantibodies detects both thyroid‐blocking and stimulating activity

Summary Autoantibodies to the thyrotrophin ( TSH ) receptor (anti‐ TSHR ) are unique, in that they are involved directly in the pathophysiology of certain autoimmune thyroid diseases ( AITD ). Thyroid‐stimulating antibodies ( TSAb ) act as agonists that activate the thyroid gland and cause G raves' disease. Other anti‐ TSHR antibodies block TSH and can cause hypothyroidism. Thyroid‐blocking antibodies ( TBAb ) have not been studied as extensively as TSAb . We developed a TBAb bioassay based on a cell line that expresses a chimeric TSHR . The 50% inhibitory concentration of the chimeric Chinese hamster ovary ( CHO) ‐Luc cells was more than five‐fold lower compared with the wild‐type CHO ‐Luc cells. We tested the performance of this bioassay using a thyroid‐blocking monoclonal antibody K 1‐70, established an assay cut‐off and detected TBAb in 15 of 50 (30%) patients with AITD . Interestingly, the assay detects both TSAb and TBAb and measures the net activity of a mixture of both types of antibodies. There was a high correlation ( R 2 0·9, P  < 0·0001) between the results of the TSAb assay and the negative percentage inhibition of the TBAb assay. The TBAb bioassay was approximately 20‐fold more sensitive than a commercially available TSHR binding assay ( TRAb ). In contrast to TRAb , sera with high levels of TBAb activity were able to be diluted several hundred‐fold and still exhibit blocking activity above the cut‐off level. Thus, this TBAb bioassay provides a useful tool for measuring the activity of anti‐ TSHR antibodies and may help clinicians to characterize the diverse clinical presentations of patients with AITD .