Objectives: I-123 is considered an ideal isotope for diagnostic studies. A significant advantage of I-123 over I-131 is that, the radiation dose delivered is about 100-fold less. Consequently, potential ‘‘stunning'' because of the dosimetry procedure prior to treatment would be less concerning. However, its relatively short half-life makes it impractical for a prolonged bio kinetic study. The aim of the current study was to assess the relationship between actual I-131 retention and the success rate of I-131 ablation in patients with differentiated thyroid carcinoma (DTC) following I-123 whole body scan (WBS). Patients and methods: 23 consecutive patients with newly diagnosed DTC (21 papillary, and 2 Follicular), underwent total or near total thyroidectomy, followed by I-123 WBS, and subsequent I-131 ablation were enrolled in current study. The whole body external exposure rate from the patient at 1 meter was measured immediately after ingestion of I-131, after 24 and 48 hours, using a calibrated Victoreen 451P ionization chamber survey meter (Fluke Biomedical, USA). The retained activity at 48h and the whole body effective half-life for each patient were then calculated. Patients were classified according to their follow up data after 6 months into two groups; those who have successful ablation (Negative WBS and an undetectable serum thyroglobulin level in the absence of antithyroglobulin antibodies), and those who need reablation (Positive WBS and/ or elevated serum Thyroglobulin). Results: Of the 23 patients studied, 15 were considered to have successful ablation and 8 still need re-ablation. No significant difference regarding age, gender, histopathology, and activity of I-131 ablation dose between both groups. The retained I-131 activity at 48 hours was 13.50±3.87% of the administered dose, and the effective half- life was 16.37±2.43 hours for patients who have successful ablation compared to 7.79±3.28% (p =0.002), and 12.64±2.27 hours (p = 0.002) respectively for patients who need re-ablation. Conclusions: We found a significantly higher retained I-131 activity at 48 hours and longer effective half-life among patients successfully ablated compared to those who need re-ablation, and recommend dosimetry adjusted I-131 ablation dose whenever feasible accordingly.