TSH receptor (TSHR) autoantibody (TRAb) is the serological hallmark of Graves' disease (GD). Third-generation enzyme-linked immunosorbent assays (ELISAs) using monoclonal TRAbs instead of TSH have been found useful for TRAb analysis recently. For the first time, a mouse monoclonal antibody (mAb) against TSHR was analyzed for TRAb detection and compared with human mAb M22 and TSH by the same competitive binding assay technique. A mouse monoclonal antibody (T7) binding to the TSH receptor and inhibiting TSH binding was generated and used for TRAb analysis in a third-generation ELISA. Obtained TRAb levels were compared with a second-generation TRAb assay employing bovine TSH and a third-generation assay with human mAb M22 as TSHR-binding reagents by investigating 89 patients with GD, 56 with Hashimoto's thyroiditis (HT), 73 with non-autoimmune thyroid diseases, 17 with rheumatoid arthritis, and 100 healthy subjects. The T7-based TRAb ELISA did not reveal a significantly different assay performance (area under the curve [AUC]) in contrast to the TSH and M22-based TRAb ELISAs by receiver operating characteristic (ROC) curve analysis (AUC-T7 0.967, AUC-TSH 0.972, AUC-M22 0.958, p > 0.05, respectively). After adjustment of cutoffs by ROC, all three TRAb ELISAs demonstrated sensitivities and specificities above 89.9% and 96.0%, respectively. Both third-generation TRAb ELISAs showed a tendency for a higher prevalence of TRAb positives in HT in contrast to the second-generation ELISA. Mouse mAbs against the TSHR may be used for the reliable detection of TRAb by third-generation TRAb ELISA. The earlier reported higher sensitivity of third-generation TRAb ELISA in GD needs to be considered in the context of a slightly lower specificity regarding HT.