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Objective: We aimed to evaluate the discordance of 5 mCi (185 MBq) I-131 whole body scan (WBS) and thyroglobulin (Tg) values at the ablation outcome control in differentiated thyroid cancer (DTC) patients who had thyroidectomy and then received radioiodine (RAI) ablation. Materials and Methods: We retrospectively evaluated 36 DTC patients who had RAI treatment in our department between 1992-2009 and whose 5 mCi (185 MBq) I-131 WBS were negative, but Tg values were ≥2 ng/ml during the ablation outcome control (Patient group). Thirty-six patients whose Tg values were < 2 ng/ml and showed no discordance at the same control made up the control group. Patient and control groups were compared in terms of age, gender, histopathological features, ablation dose, Tg value before ablation, and 24. hour RAI uptake value during ablation. The patient group was then evaluated for the cause of the discordance. Results: There were 28 female and 8 male patients whose mean age was 45.6±11.39 in patient group. In the control group, there were 29 female and 7 male patients whose mean age was 41.5±11.69. According to the reason of discordance at the ablation outcome control, the patient group was divided into 2 groups: 15 (42%) patients (9 female, 6 male patients, mean age: 50.66±10.73) who had metastatic lymph nodes as the reason of discordance constituted the lymph node group. The remaining 21 (58%) patients (19 female, 2 male patients, mean age: 41.5±10.44) were the micrometastatic group in which the cause of discordance could not be determined. There were statistically significant differences in soft tissue invasion (p2 cm (p2 cm (p40 (p