Objective: Differentiated thyroid cancer (DTC), the most common subtype of thyroid cancer, has a relatively good prognosis. The 8th edition of the American Joint Committee on Cancer (AJCC) pathologic tumor-node-metastasis (T [primary tumor size], N [regional lymph nodes], M [distant metastasis]) staging system did not take the T stage into consideration in stage IV B DTC patients. We evaluated the prognostic value of the T stage for advanced DTC survival. Methods: DTC cases that were considered stage IV B in the AJCC 8th edition were extracted from the Surveillance, Epidemiology, and End Results database. T stage (AJCC 6th standard) was categorized into T0-2, T3 and T4. We analyzed overall survival (OS) and cancer specific survival (CSS) in the overall group as well as in pathologic subgroups. We used the Kaplan-Meier method and log-rank test for univariate analysis and the Cox regression model for multivariate analysis. Results: A total of 519 cases were extracted. Patients with earlier T stages showed significantly better OS and CSS in univariate analysis. T stage was an independent prognostic factor for both OS and CSS in multivariate analysis. Subgroup analysis in papillary and follicular thyroid cancer showed that T4 was an independent prognostic factor for both OS and CSS. Conclusion: AJCC 8 stage IV B DTC patients could be further stratified by T stage. Further studies with larger samples and AJCC 8 T stage information are necessary. Abbreviations: AJCC = American Joint Committee on Cancer; CI = confidence interval; CSS = cancer specific survival; DTC = differentiated thyroid cancer; FTC = follicular thyroid cancer; FVPTC = follicular variant of papillary thyroid carcinoma; HR = hazard ratio; OS = overall survival; PTC = papillary thyroid cancer; SEER = surveillance, epidemiology, and end results database.