Differentiated thyroid carcinomas today account for 90% to 95% of all malignant thyroid changes, and in a large number there is an indication for performing ablation with various therapeutic activities of I-131. The study included a consecutive series of 50 patients with well-differentiated thyroid cancer who underwent total thyroidectomy and performed ablation with I-131 activity of 30 mCi. In the examined patients the disease was limited to the thyroid gland without spreading to the surrounding structures (T1-T3), without distant dissemination of the disease (M = 0) and without metastases to regional lymph nodes (N = 0) were 49 observed patients and one patient has metastases at the level of VI lymph nodes (N1a). After ablation, all patients underwent I-131 scintigraphy and serum thyroglobulin (Tg) levels were measured. The aim of this study was to determine which is more sensitive to determine the success of radioiodine ablation - scintigraphy with I-131 or serum thyroglobulin (Tg) levels. For the purposes of this paper, the databases of the Clinical Institute of Nuclear Medicine of the Clinical Hospital Center Split in the period from 2017 to 2018 were accessed. Methods of graphical and tabular representation were used, which present the structures of the observed patients according to the observed characteristics. Numerical values (median and interquartile range) were presented using descriptive statistics methods. The set goals were examined by x2 test with Yates correction and with sensitivity calculation. The analysis was performed in the statistical software STATISTICA 12.In the observed group, the representation of women is higher than that of men, and the mean age is 49.50. No association was found between sex and age with the obtained I-131 scintigraphy findings or with the measured serum thyroglobulin (Tg) level. The first hypothesis test revealed more frequent negative Tg levels in negative scintigraphy findings compared to positive scintigraphy findings and also found an association between postablastic scintigram and TgAt levels as well as higher Tg sensitivity relative to TgAt. Repeated hypothesis testing shows that there is no difference between serum Tg levels and scintigraphy findings and that all patients scintigraphed with I-131 have a TgAt value of less than 100.