Simple SummaryThe aim of the present study was to retrospectively evaluate the efficacy of low (1.1 GBq) versus moderate (2.2 GBq)131I activities in a large series (n = 299) of low-risk differentiated thyroid carcinoma (DTC) patients requiring postoperative131I ablation. At the follow-up, according to the ATA criteria, an excellent response was observed in 96.9% of patients treated with moderate131I activities versus 85.6% of patients treated with low131I activities (p= 0.029). Conversely, a biochemically indeterminate or incomplete response was observed in 22.2% of patients treated with low131I activities versus 1.8% of patients treated with moderate131I activities (p= 0.001), and an incomplete structural response was observed in three patients treated with low131I activities versus two patients treated with moderate131I activities (p= 0.654). In conclusion, we encourage the use of moderate instead of low activities when131I ablation is indicated in order to reach an excellent response in a significantly larger proportion of patients, including patients with the unexpected persistence of the disease.AbstractObjectives: To compare the efficacy of low and moderate131I activities in low-risk differentiated thyroid carcinoma (DTC) patients requiring postoperative thyroid remnant ablation in a real-world clinical setting. Methods: We retrospectively reviewed the records of 299 low-risk DTC patients (pT1-T2, Nx(0) Mx) who had undergone (near)-total thyroidectomy followed by131I therapy, using either low (1.1 GBq) or moderate (2.2 GBq) radioiodine activities. The response to initial treatments was evaluated after 8–12 months, and patient responses were classified according to the 2015 American Thyroid Association guidelines. Results: An excellent response was observed in 274/299 (91.6%) patients, specifically, in 119/139 (85.6%) and 155/160 (96.9%) patients treated with low and moderate131I activities, respectively (p= 0.029). A biochemically indeterminate or incomplete response was observed in seventeen (22.2%) patients treated with low131I activities and three (1.8%) patients treated with moderate131I activities (p= 0.001). Finally, five patients showed an incomplete structural response, among which three and two received low and moderate131I activities, respectively (p= 0.654). Conclusions: When131I ablation is indicated, we encourage the use of moderate instead of low activities, in order to reach an excellent response in a significantly larger proportion of patients, including patients with the unexpected persistence of the disease.