LONG-TERM EFFECTS OF RADIOIODINE ON THYROTROPIN RECEPTOR ANTIBODIES IN GRAVES-DISEASE

OBJECTIVE Graves' disease Is recognized as an organ-specific autoimmune disorder caused by the presence of TSH receptor antibodies. The long-term effects of I-131 treatment for Graves' disease on TSH receptor antibodies have not previously been studied. We have measured the TSH-binding inhibitory immunoglobulin (TBII) Index and thyroid stimulating antibody (TSAb) activity in patients with Graves' disease following treatment with I-131. DESIGN A retrospective study. PATIENTS Two hundred and twenty-five patients with Graves' disease who were treated with I-131 1-13 years earlier were studied (1 year:27 patients; 2-5 years:42 patients; 6-9 years:79 patients; 10-13 years:77 patients). MEASUREMENTS The TBII index was measured as the percentage I-125-TSH bound to pig thyroid membranes and TSAb activity as the amount of cAMP produced by cultured FRTL-5 cells. RESULTS TBII was detected in 78% of patients prior to I-131 administration. Following I-131 administration, the Incidence of positive Tail was 85% at the end of the first year decreasing to 40, 19 and 17% at 2-5, 6-9 and 10-13 years, respectively. The frequency of a positive TSAb was 74% at the end of the first year, and also decreased to 49, 27 and 29% at 2-5, 6-9 and 10-13 years, respectively. At more than 2 years after I-131 therapy, the frequencies of hyperthyroidism in Tall and TSAb positive patients were 42% (19/45) and 30% (19/63), respectively, which were significantly higher than those in TBII and TSAb negative patients (8%:12/153 and 8%:11/131, respectively). The frequency of hyperthyroidism after I-131 treatment in patients with negative Tail before treatment (7%:2/29) was significantly lower than that (29%:30/102) in patients with positive TBII before treatment. CONCLUSIONS These results indicate that (1) the Tall index and TSAb activity decreased over a period of more than 2 years after I-131 therapy for Graves' disease, and (2) the Tell index before treatment may influence the longterm outcome of I-131 therapy.