BACKGROUND: TSH receptor blocking antibody (TRBAb) is a pathogenic factor in the vast majority of patients with primary myxedema. It has been reported that TRBAbs are found in some patients with chronic goitrous autoimmune thyroiditis (Hashimoto's thyroiditis), but the significance or the role of TRBAb in Hashimotos thyroiditis is not clear, We recently reported that hTSHR-CHO cells which express the functional human TSH receptors are more sensitive and are better in detecting functional TSH receptor antibodies in Graves patients than FRTL-5 cells. We are to investigate the biological role of TRBAb in Hashimotos thyroiditis by measuring thyroid stimulation blocking antibody (TSBAb) activities of Hashimoto's IgG's using hTSHR-CHO cells. Moreover, we are to see if there is any difference in epitope recognition between Hashimotos TRBAb and myxedema's TRBAb by measuring TSBAb activities with mutant receptor expressing cell lines, Mcl+2 and Mc 2 in those patients. METHOD: We measured TSBAb activities of IgGs from patients with primary myxedema (PM, n= 10) and those with hypothyroid (n 20) or euthyroid (n 17) Hashimoto's thyroiditis (HT) using wild type hTSHR-CHO cells (WT) and two chimeric receptor expressing cell lines, Mcl+2 and Mc2. RESULTS: TSBAb activities measured by WT were higher in hypothyroid HT than in euthyroid HT (30.0+-23.2% vs. 6.1+-28.7, p<0.05), and TSBAb-positive rate tend to be higher in the former (20%, 5/20) than in the latter (0%, 0/17, p=0.07). TRBAbs from PM (n=4) had high TBII activities and had persistent blocking activities despite of the replacement of amino acid residue 8~165 of extracellular domain of TSHR with those of rat LH/CGR (Mcl +2). However, TRBAbs from HT (n=4) had no TBII activity at all and lost blocking activities when measured with Mcl+2. CONCLUSION: TRBAbs are found in 20% of hypothyroid patients with Hashimotos thyroiditis in assay using hTSHR-CHO cells, and they seem to play a role in the development of hypothyroidism in some patients with Hashimotos thyroiditis. TRBAbs of Hashimotos thyroiditis are different in epitope recognition from TRBAbs of primary myxedema.