BACKGROUND: The Chinese hamster ovary cells transfected with human TSH receptor cDNA (hTSHR-CHO), expressing functional human TSH receptors, are known to be more sensitive in detection of thyroid stimulating antibodies than FRTL-5 cells. There has been no report on the usefulness of these cells to measure thyroid stimulation blocking antibody (TSBAb) activity which is frequently found in patients with primary myxedema, METHODS: We established the optimal assay condition of measurement of TSBAb using hTSHR-CHO cells, and simultaneously measured TSBAb activities with FRTL-5 cells and with hTSHR-CHO cells in 49 patients with primary myxedema, compared them with their thyrotropin binding inhibitor immunoglobulin (TBII) activities. RESULTS: 1) hTSHR-CHO cells specifically bound bTSH and were stimulated by bTSH in terms of cyclic AMP generation in a dose dependent manner. 2) Myxedema IgG suppressed TSH-stimulated cAMP production of hTSHR-CHO cells in a dose dependent manner reaching plateau at the concentration of I g/L. Normal pooled IgG has no suppressive action at the concentration of less than 1 g/L, but caused significant suppression at the concentration of greater than 1g/L. 3) TSBAb activities measured by hTSHR-CHO cells in 49 patients with primary myxedema were as follows: Four of 25 TBII-negative cases (16%) and 22 of 24 TBII-positive cases (92%) had TSBAb activities. Most of TSBAb positive patients (95%), especially in TBII positive cases, showed very high activities of more than 90%. 4) TSBAb activities measured by hTSHR-CHO cells and those by FRTL-5 cells were both positive in 24 patients (49%), both negative in 18 patients (37%), and were discrepant in 7 patients (14%). The TSBAb activities measured with hTSHR-CHO cells and those measured with FRTL-5 cells were significantly correlated (r=0.71, p< 0.01). 5) Forty five percent of patients with primary myxedema had all of 3 kinds of activities (TBII, hTSHR-CHO cell TSBAb, FRTL-5 cell TSBAb), 37% of them had none of 3 activities and 18% of them had 1 or 2 kinds of activities only. CONCLUSION: The usefulness of hTSHR-CHO cells in measurements of TSBAb activities were confirmed. The TSBAb activities of most patients with primary myxedema measured by hTSHR-CHO cells were concordant with those measured by FRTL-5 cells. However, a small subset of patients (18%) had discrepant results in assays of TSH receptor antibodies according to the differences in TSH receptors (rat, human and porcine) used in assay. Such discrepancy may be explained by heterogeneity in epitopes for blocking TSH receptor antibodies.