Differences Between Changes in Serum Thyrotropin-Binding Inhibitory Antibodies and Thyroid-Stimulating Antibodies in the Course of Antithyroid Drug Therapy for Graves' Disease

There has recently been controversy regarding whether the measurement of thyrotropin-binding inhibitory antibodies (TBIAb) is useful in the management of Graves' disease. Another method of assessing Graves' disease by measuring adenylate cyclase activity in thyroid cells, known as thyroid-stimulating antibodies (TSAb), differs from TBIAb not only in terms of assay but also in immunoglobulin type according to recent studies. In this study, the concentrations of TBIAb and TSAb were compared in serial serum samples collected from 29 patients with Graves' hyperthyroidism during 12 months of antithyroid drug therapy. Before therapy, there was a correlation between TBIAb and TSAb (r = 0.59). The radioactive iodine uptake (RAIU) was not significantly correlated with either TBIAb or TSAb (r = 0.20 and r = 0.29, respectively), and the serum free thyroxine (FT₄) concentration was also not significantly correlated with either TBIAb or TSAb (r = 0.06 and r = 0.22, respectively). In patients with Graves' ophthalmopathy, TSAb levels were higher than in patients without ophthalmopathy (1015% ± 851% vs. 456% ± 323%, p < 0.01), but the TBIAb levels were not significantly different. After antithyroid treatment, TBIAb did not decrease significantly (from 42.1% ± 20.8% to 20.5% ± 19.5%, p = 0.29). On the other hand, TSAb was significantly decreased after 12 months of treatment (from 649% ± 611% to 294% ± 205%, p < 0.05). These findings indicate that TBIAb and TSAb are not identical, and that TSAb has a closer relationship to thyroid function than TBIAb. In the clinical setting, determination of the serum TSAb level may provide a more accurate index of the thyroid status in Graves' disease patients receiving antithyroid therapy.