At attempt to detect the auto-immune origin of hyperthyroidism may be made by measuring thyrotropin binding inhibitory immunoglobulin (TBII) which uses the antibodies ability to inhibit labelled TSH binding to the TSH receptor. This study was carried out on 196 patients of which 128 had autoimmune hyperthyroidism and 68 another thyroid disease. Construction of receiver operating characteristic (ROC) curves allowed us to show the quality of the assay. This method, according to the prevalence of the disease, helped to determine the ideal cut-off of the assay. This cut-off was between 13.5 and 7.5% for prevalences ranging between 10 and 80%. For a 9% cut-off, which corresponded to the group studied, we observed 87.5% sensitivity and 87% specificity. Existence of false positives and false negatives was linked to the assay method which only informed us about the occupation of the TSH binding site and not its physiological activity. However, we concluded this easy to perform assay is a good test for the diagnosis of autoimmune hyperthyroidism.