High prevalence and little change in TSH receptor blocking antibody titres with thyroxine and antithyroid drug therapy in patients with non-goitrous autoimmune thyroiditis

OBJECTIVE We have reevaluated the prevalence and pathogenetic importance of TSH receptor blocking antibodies (TRBAb) in autoimmune hypothyroidism, and investigated the changes in TRBAb activities during thyroxine and antithyroid drug treatment. DESIGN Serum TSH binding inhibitor immunoglobulin (TBII) and thyroid stimulation blocking antibody (TSBAb) were measured serially in all patients with non-goitrous autoimmune thyroiditis (AT) and measured monthly during methimazole treatment in 6 patients. PATIENTS Ninety patients with non-goitrous AT and 95 patients with goitrous AT were entered consecutively into this study. All patients with non-goitrous AT were treated with thyroxine and followed at intervals of 6 months for 2 years initially and then yearly intervals. The duration of follow-up was 1–8 years. Six patients from the TRBAb-positive non-goitrous AT group who were treated with thyroxine were randomly selected and given additional treatments with methimazole (40 mg per day) for 6 months. MEASUREMENTS Serum TBII was measured by 8 radioreceptor assay, TSBAb by using FRTL-5 cells, and antithyroid peroxidase and antithyroglobulin antibodies by radioimmunoassay. RESULTS The prevalences of TBII and TSBAb in non-goitrous AT were 47–8 and 58.9%, respectively, which were significantly higher than those in goitrous AT (6.3% for TBII, 10.5% for TSBAb). All but one patient showed persistent TBII and TSBAb activities during the thyroxine treatment for up to 8 years. A high dose of methimazole (40 mg per day) did not affect the titres of TBII and TSBAb in 5 out of 6 patients with non-goitrous AT tested. However, antithyroid peroxidase and antithyroglobulin antibodies activities were significantly decreased during the methimazole treatment. CONCLUSION The high prevalence of TSH receptor blocking antibodies (TRBAb) suggests that TRBAb may play a major role in the development of hypothyroidism and thyroid atrophy in the vast majority of patients with non-goitrous autoimmune thyroiditis. Most TRBAb activities are stable for at least 8 years and are not affected by thyroxine and antithyroid drug treatment.