The radioiodine therapy can favour and induces of autommunological reaction in thyroid gland. The aim of the study was evaluation of antithyroid autoantibodies in patients with multinodular large toxic goiter treated with repeated doses of 131I before and after therapy. Studies were conducted in 24 women (age range: 65-84 yrs) with multinodular large toxic goiters--goiter volume assessed by USG over 100 ml. Serum TSH, fT4, antithyroid antibodies (anti-TPO, anti-Tg, TSHR-Ab) levels were estimated in all patients parallel with radioiodine uptake test (after 5 and 24 hours), 131I thyroid scintigraphy and fine needle biopsy. These studies and therapy with 22 mCi 131I were repeated every 3 months. Before therapy median of thyroid volume was approximately 195 ml and during therapy gradually decreased to 110 ml after 12 months. After 12 months we found 11% of patients with hyperthyrodism, 62% of patients with euthyroidism and 27% of patients with hypothyroidism. Before radioiodine treatment aTg and aTPO presence were detected in the most of patients, but only in 5 cases above normal value. TSHRAb were detected (normal, very low values) in 16 patients. During therapy statistically significance increase of TSHRAb levels (median: from 0,27 to 0,65 after 6 months and to 0,71 IU/l after 9 months) was observed; aTPO and aTg antibodies levels showed marked tendency to rise, but without significant differences (aTPO median: from 40 IU/ml to 48 IU/ml; aTg - median: from 27 IU/ml to 46 IU/ml). During these observations we didn't find evident correlation between the levels of antithyroid antibodies, radioiodine uptake, proved reduction of goiter volume and TSH, FT4, FT3 values.