Characteristics of anti-TSH receptor antibodies in two patients who developed spontaneous hypothyroidism after antithyroid drug therapy for hyperthyroid Graves' disease.

Spontaneous hypothyroidism was developed in two patients with Graves' hyperthyroidism 1.8 or 1 years after antithyroid drug therapy. In these patients anti-thyrotropin (TSH) receptor antibodies were detected. Thyrotropin receptor antibody (TRAb) index measured with TSH radioreceptor assay was 61.0% in case 1 and 68.0% in case 2 (normal value: -10 to 10%), and human thyroid adenylate cyclase stimulating activities (HTACS) were 154% in case 1 and 190% in case 2 (normal value: 70 to 130%). IgG from case 1 suppressed the stimulating activity of bovine TSH (bTSH) on thyroid adenylate cyclase in low concentrations in vitro. These results indicate that a blocking type of anti-TSH receptor antibodies as well as concomitant chronic thyroiditis might be a causative factor in patients who developed spontaneous hypothyroidism shortly after the cessation of antithyroid drugs.