GOAL OF STUDY: Investigation of radio-iodine ablation of the residual thyroid tissue after a total thyroidectomy with the ablation dose of 30 mCi J-131 in patients operated for well-differentiated carcinoma measuring up to 4 cm, without extra-thyroid infiltration and without metastases, and to compare the results with the ablation dose of 100 mCi J-131. HYPOTHESIS: The total activity of J-131 after the successfully conducted ablation is going to be smaller in patients who received an ablation dose of 30 mCi J-131 than in patients who were treated with ablation dose of 100 mCi J-131. In most patients who received a successful ablation dose of 100 mCi J-131 the same results could be received with a dose of 30 mCi J-131. Higher sensitivity level in detection of the residual thyroid tissue is seen with scintigram (J-131) than with thyroglobulin. SUBJECTS AND METHODS: I’ve conducted a retrospective observation research in the Clinical department for nuclear medicine, Clinical hospital centre Split, in the period from 2008.-2010., by investigating the medical documentation of 108 test subjects which have received an ablation dose of 30 mCi J- 131 and of 113 test subjects which were given the ablation dose of 100 mCi J-131. Second success parameter of the ablation in previously named groups of subjects was serum concentration of Tg before and after the successful ablation was conducted. RESULTS: Of 108 test subjects which received the ablation dose of 30 mCi J-131, successful ablation was observed in 85 (79%) of the subjects while in 113 who were given the ablation dose of 100 mCi J-131 showed success in 91 (89%) test subjects. The average activity of successful ablation in a group of subjects which were given the ablation dose of 30 mCi J-131 was 41 mCi, which is 2,6 times smaller than the average dose in the group of subject which were given the ablation dose of 100 mCi J-13, which was 107 mCi J-131. Higher sensitivity in detection of the residual thyroid tissue is seen in a scintigram with J-131 than with thyroglobulin (100% vs. 56%). In fact, Tg could have been used only in 125 patients because 45 of the test subjects had positive thyroglobulin antibodies and 51 subject,with negative Tg antibodies, had a negative Tg result at the time of the ablation. The success of the ablation was measured with the negative Tg results comparison which didn't differ in between the two ablation doses (79% in the group which was given the ablation dose of 30 mCi and 84% in the group which was given the ablation dose of 100 mCi) neither did the average reduction of Tg (0,8 vs. 0,7 ng/ml). Higher sensitivity in detection of the residual thyroid tissue was seen in a scintigram with J-131 than in thyroglobulin (100% vs. 56%). CONCLUSION: Although the success of ablation with the ablation dose of 30 mCi J-131 was 10% smaller than the success with the ablation dose of 100 mCi, the average activity of successful ablation was 2,6 times smaller with the ablation dose of 30 mCi J-131 than with the ablation dose of 100 mCi J-131. In 79% of test subjects which went through a successful ablation with a dose of 100 mCi J-131 the same efficiency could have been conducted with a smaller ablation dose of 30 mCi J-131. Higher sensitivity in detection of the residual thyroid tissue was seen in a scintigram with J-131 than in thyroglobulin (100% vs. 56%). The success of ablation measured by comparing the negative thyroglobulin results doesn’t differ in between the two ablation doses but the result can be only used in 56% of test subjects.