Differences Between Thyrotropin Receptor Antibody Bioactivity and Inhibition of¹²⁵l-Bovine Thyrotropin Binding

Abstract

Thyrotropin (TSHR) receptor antibodies that bind to the TSHR without stimulating the TSHR have been identified with a direct binding assay. Moreover, TSHR antibodies that exhibit thyroid epithelial cell stimulation without inhibition of 125I-bovine thyrotropin (bTSH) binding and vice versa have been described. These data suggest that stimulation or blocking of the TSHR by stimulating (TSAB) or blocking (TSBAB) TSHR antibodies could be possible without detectable bTSH-displacement activity. However, to date, possible differences between TSAB or TSBAB activity and inhibition of 125I-bTSH binding have not been systematically investigated. Therefore we compared inhibition of 125I-bTSH binding and TSAB or TSBAB activity of sera from 113 patients with Graves' disease treated with antithyroid drugs. To exclude the different assay conditions of previous investigations as possible confounding factors, we determined TSAB or TSBAB and inhibition of 125I-bTSH binding (TBIIW) with the same Chinese hamster ovary (CHO) cells expressing the human TSHR. Furthermore inhibition of 125I-bTSH binding was also determined as thyrotropin-binding inhibitory immunoglobulin (TBII) with solubilized porcine thyroid membranes (TRAK, Brahms, Berlin Germany) and the highly sensitive recombinant human TSH receptor assay (hTRAK, Brahms, Berlin Germany). Only 78% (54/69) of TSAB-positive and 78% (21/27) of TSBAB-positive sera detected with JP26 cells exhibit inhibition of 125I-bTSH binding measured as TBII or TBIIW. Furthermore, 59% (10/17) of sera without TSAB and TSBAB activity revealed inhibition of 125I-bTSH binding measured as TBII or TBIIW. We found significant differences between TSHR bioactivities (TSAB or TSBAB) and inhibition of 125I-bTSH binding. Moreover, there was no agreement between the detectable TSHR bioactivities (TSAB or TSBAB) and their detectable inhibition of 125I-bTSH binding. Therefore, it is very likely that TSH displacement by TSHR antibodies and stimulation or blocking of the TSHR by TSHR antibodies are different functions that do not need to occur together.

TSH‐receptor antibody measurement for differentiation of hyperthyroidism into Graves' disease and multinodular toxic goitre: a comparison of two competitive binding assays

“…TSH receptor antibodies may be heterogeneous with respect to species specificity, as demonstrated in the present study, but also in their ability to bind, stimulate or block the TSH receptor (Wallaschofski et al ., 2000). At present, only competitive binding assays, such as those evaluated in the present study, are generally available for diagnostic purposes.…”

Induction of TSH-Receptor Antibodies in Patients with Toxic Multinodular Goitre by Radioiodine Treatment

“…All 4 patients with detectable TBII revealed neither TSAB nor TSBAB. The determination of TSAB and TSBAB has been performed like previously reported [12]. In a previous study, we systematically investigated the influence of different assay parameters on the sensitivity of the TSAB/TSBAB CHO cell bioassay.…”

“…TRAbs were determined as TSAB or TSBAB with the JP26 bioassay and as TBII with the DYNOtest TRAK human assay (Brahms, Berlin, Germany) as previously reported [12]. Anti-TPO-Abs and anti-TG-Abs were analysed using the DYNOtest anti-TPO assay (BRAHMS, Berlin, Germany) and the DYNOtest anti-TG assay (Brahms, Berlin, Germany) respectively, performed according to the manufacturer's instructions.…”

Therapy of Hyperthyroidism

“…At withdrawal of antithyroid drug treatment TSBAb were detectable in 40 % (Wallaschofski and and 12% of patients (Zhu et al, 1994), respectively. Parallel determinations of TBII, TSAb, and TSBAb in sera of 113 patients with Graves disease under identical assay conditions suggest that stimulation or blocking of the TSHR by TSHR antibodies could be possible without detectable bTSHdisplacement activity (Wallaschofski et al, 2000). Moreover, we measured the changes of TBII, TSAb, and TSBAb in 54 patients with Graves disease at the end of ATDT and 12.5 months after cessation of ATDT (Wallaschofski et al, 2002).…”