MON-591 Thyroid Function and Autoimmunity Status after One and Three Months of Radioactive Iodine Therapy for Thyrotoxicosis

Background Radioioactive iodine therapy (RAI) is an excellent choice to treat thyrotoxicosis, particularly Graves´ disease (GD) patients. TSH receptor antibodies (TRAb) rise in GD patients and autoimmunity can surge after RAI. Objective Evaluate thyroid hormone function and autoimmunity 1 and 3 months after RAI therapy for thyrotoxicosis. Patients and Methods Patients were evaluated prospectively before and 1 and 3 months after RAI therapy for thyrotoxicosis. Thyroid hormones, TPO antibody (TPOAb) and thyroglobulin antibody (TgAb) were all measured using immunoassays (Roche Diagnostics Ltd). TRAb was determined by electrochemiluminescence assay (Cobas, Roche Diagnostics Ltd) with TRAb positive values >1.75 IU/L. Free T3 (FT3) and thyroid stimulating immunoglobulins (TSI) were determined by chemiluminescent immunoassays (Immulite, Siemens Healthcare Diagnostics) with TSI positive > 0.55 IU/L. All patients stopped antithyroid medications 2 to 4 weeks before RAI uptake and treatment and low iodine diet was recommended. Antithyroid medications were not introduced before 1 month. Results A total of 53 patients was prospectively evaluated (10 males), with mean age 46.7 ± 0.41 years-old (25 to 82). GD was the most frequent etiology of hyperthyroidism (83%), followed by toxic multinodular goiter (TMNG) (11%) and toxic adenoma (TA) (6%). TMNG and TA patients had no thyroid autoimmunity. TRAb was positive in 91% and TSI 92% among GD. TPOAb and TgAb were positive in 80% and 48%, respectively in GD patients. RAI uptake at 2 and 24 hours were high in all patients (38.4%±46% and 67%±34%, respectively). A single oral RAI was given with X=22±7.8 mCi (15 to 34 mCi). Only one GD patient developed hypothyroidism at 1 month, meanwhile 58% and 10.5 % were still in hyperthyroidism after 1 and 3 months, respectively. After RAI, none but one GD patient developed positive thyroid antibody (only TPOAb). TRAb levels significantly increased from baseline in 38% and 58% of GD patients after 1 and 3 months, respectively as well as TSI levels (48% and 64%). Conclusions RAI therapy is effective to treat thyrotoxicosis but hyperthyroidism is not achieve in short term. It is also safe as autoimmunity is rarely triggered but caution must be taken in those patients with pre-existent autoimmunity, as all thyroid antibodies can increase after this therapy.