This study aimed to assess recurrence-free survival (RFS) rates and recurrence-related factors of patients who received adjuvant therapy (AT) with radioactive iodine (RAI) for differentiated thyroid cancer (DTC) following thyroidectomy. We evaluated 284 patients who underwent AT between January 2011 and July 2020 at our hospital. Recurrence was defined as visible recurrent lesions on image analysis or need for repeat surgery with pathologically confirmed recurrent lesions. RFS rate and prognostic factors were statistically evaluated. The median observation period was 30.2 months (range, 5.7–294 months). Overall, 192 patients were female and 92 were male, and the median age was 54 years (range, 9–85 years). Initial assessment revealed 39 recurrence cases. The 3-year RFS rate was 85.8% (95% confidence interval: 81.1–90.9%). Univariate analysis revealed that histology (except for papillary carcinoma), Tg level > 4 ng/dL before AT, and AT result significantly exacerbated the RFS rate. In multivariate analysis, histology and AT result were also important contributors to the worsening RFS rate. Results of AT can be determined relatively early and are important in predicting future recurrence in patients with DTC. Increasing the success rate of AT may lead to an improved prognosis.