Effect of radioiodine treatment for Graves' disease on the generation of TSH anti-receptor stimulating antibodies (TSI).

OBJECTIVE: It is well established that serum levels of TSH receptor antibodies (TRAb) rise after radioiodine (131I) therapy for Graves' disease (GD). However, it remains unclear whether these post-therapy autoantibodies are predominantly TSH receptorstimulating immunoglobulins (TSI) and how their persistence might affect treatment outcomes. SUBJECTS AND METHODS: In this prospective study, 39 patients with GD underwent 131I therapy. Serum TRAb (measured by competitive electrochemiluminescence, ECLIA) and TSI (measured by an IMMULITE® 2000/2000 XPi TSI assay) were evaluated at baseline and at 1, 2, 3, 6, 9, and 12 months post-therapy. More than 7% increase from baseline was considered a significant rise. RESULTS: At diagnosis, all 39 patients tested positive for TRAb, while 38 tested positive for TSI. Both TRAb and TSI levels rose significantly between months 2 and 4 post-131I, followed by a progressive decline by months 9 to 12. TSI increased in 72% of patients; of these, 93% showed a gradual decrease but remained higher than baseline in 58% at 12 months. Patients with thyroid eye disease (TED), longer disease duration, or higher baseline TSI more frequently exhibited persistent elevation at one year. Despite the persistence of TSI, all patients achieved control of thyrotoxicosis (euthyroid or hypothyroid states). CONCLUSION: Radioiodine therapy leads to an increase in TSI, which can remain elevated for up to 12 months in more than half of GD patients. These findings suggest potential benefits of measuring TSI for guiding management decisions, particularly regarding antithyroid drug discontinuation and pregnancy planning.