Radioactive iodine (RAI) is a key component in the treatment of differentiated thyroid cancer. RAI has been recommended more selectively in recent years as guidelines evolve to reflect risks and utility in certain patient subsets. In this study we sought to evaluate the survival impact of radioactive iodine in specific thyroid cancer subgroups. Nationwide retrospective cohort study of patients using the National Cancer Database (NCDB) from 2004 to 2012 and Surveillance, Epidemiology, and End Results (SEER) database from 1992 to 2009 examining patients with differentiated thyroid cancer treated with or without RAI. Primary outcomes included all-cause mortality (NCDB and SEER), and cancer-specific mortality (SEER). Cox multivariate survival analyses were applied to each dataset, and in 135 patient subgroups based on clinical and non-clinical parameters. A total of 199,371 NCDB and 77,187 SEER patients were identified. RAI was associated with improved all-cause mortality (NCDB: RAI hazard ratio (HR) 0.55, P < 0.001; SEER: HR 0.64, P < 0.001); and cancer-specific mortality (SEER: HR 0.82, P = 0.029). Iodine therapy showed varied efficacy within each subgroup. Patients with high-risk disease experienced the greatest benefit in all-cause mortality, followed by intermediate-risk, then low-risk subgroups. Regarding cancer-specific mortality, radioactive iodine therapy was protective in high-risk patients, but did not achieve statistical significance in most intermediate-risk subgroups. Low-risk T1a subgroups demonstrated an increased likelihood of cancer-specific mortality with iodine therapy. The efficacy of RAI in patients with differentiated thyroid cancer varies by disease severity. A negative cancer-specific survival association was identified in patients with T1a disease. These findings warrant further evaluation with prospective studies.