Longitudinal changes in TSH receptor antibodies after Radioiodine therapy for Graves’ hyperthyroidism: relationship with thyroid weight and therapeutic efficacy

1345 Objectives: To evaluate longitudinal changes in serum TSH receptor antibodies (TRAb) following a single radioiodine therapy (RAI) for Graves’ hyperthyroidism (GH) and to correlate them with changes in thyroid weight and with therapeutic efficacy. Methods: 56 patients (Male/Female 14/42. Age 20-73yrs.) with GH who were to undergo RAI were enrolled. 53 out of 56 patients were previously treated by anti-thyroid drugs. TRAb (normal range:>2,0 IU/L) was determines by ECLIA method and was positive in all but one patients. Thyroid weight (TW) was determined using either CT or US. Anti-thyroid drugs were discontinued 3 to 4 days prior to RAI. After instruction by dietitians, patients followed a low iodine diet ( 100g. As a rule, anti-thyroid drugs were resumed on 5 days after RAI. Serum FT3, FT4, TSH, TRAb, and thyroid weight was determined over 3 years at 6, 12, 18, 24, and 36 months after RAI. Success of RAI was defined as achievement of either of euthyroid, subclinical hypothyroid, or hypothyroid status. Relationship between changes in TRAb were correlated with those in thyroid weight and success rate of RAI. Results: Pretreatment TRAb(IU/L) and TW ranged from 0,9-4.0 (Average 19.5) and17.9-127.0 (average 65.1), respectively. The average of TRAb at 6, 12, 18, 24 and 36 months was 128%(p<0.05),102%(ns), 89%(%ns), 73%(ns), and 53% (ns) of the pretreatment value, respectively. The disappearance of TRAb was seen only in 2%(1/56) of patients at 6 months after RAI. Although it significantly rose up to16% (9/56, p<0.05) at 18 months after RAI, it did not further increase at 24 and 36 months after RAI. The average of TW at 6, 12, 18, 24 and 36 months was 49% (p<0.01), 31% (p<0.01), 22% (p<0.005),18% (p<0.001), and 11% (p<0.001) of the pretreatment value, respectively. Longitudinal changes in TRAb after RAI paralleled those in TW. Overall success rate of RAI after 6, 12, 18, 24 and 36 months was 61%, 73%, 80%, 85% and 90%, respectively. There was a significant inverse correlation between longitudinal changes in TRAb and those in success rate of RAI (r=-0.99, p<0.01). Also, there was a significant inverse correlation between longitudinal changes in TW and those in success rate of RAI (r=-0.97, p<0.01). At 6 months after RAI, 55% of the patients (31/56) had TRAb values higher than 125% of the pretreatment values. However, successful rate at 6motnhs and 2 years after RAI among patients who showed elevated TRAb was comparable to that among their counterparts (65% vs. 56%, ns. and 78% vs. 81%, ns.). Conclusions: More than 50% of patients underwent RAI showed higher TRAb compared with the pretreatment value at 6months after therapy. About 80% of the patients had positive TRAb at 36 months after RAI. However, the increase in TRAb was temporally that it gradually decreased over 3 years following RAI, paralleling the longitudinal decrease in the thyroid weight. In addition, temporally elevation in TRAb or persisting positivity of TRAb did not impair efficacy of RAI for GH.