To validate the American Thyroid Association (ATA) initial risk of recurrence scheme and the Memorial Sloan Kettering Cancer Center (MSKCC) response to therapy re‐stratification approach in a large cohort of patients with differentiated thyroid cancer (DTC) treated outside of the United States.

Retrospective chart review.

Five hundred and six patients with DTC followed for a median of 10�years after total thyroidectomy and RAI remnant ablation at a major cancer centre in Brazil.

Final clinical outcomes were assessed based on American Joint Cancer Committee (AJCC)/Union Internationale Contre le Cancer (UICC) staging, ATA risk stratification and response to therapy assessment (excellent, acceptable, biochemical incomplete and structural incomplete).

The AJCC/UICC staging system did not adequately stratify patients with regard to the risk of recurrence/persistent disease. However, the ATA system demonstrated a 13% risk of recurrent/persistent disease in low‐risk patients, 36% in intermediate risk patients, and 68% in high‐risk patients. Furthermore, an excellent response to therapy decreased the risk of recurrent/persistent disease to 1�4%. At the time of final follow‐up, 34% of the biochemical incomplete response patients had been re‐classified as having no evidence of disease (NED) without having received any additional therapy beyond continue levothyroxine suppression. Conversely, even after additional therapies, only 9% of the patients with an incomplete structural response were eventually re‐classified as NED.

These data validate the ATA risk classification as an excellent initial predictor of recurrent/persistent disease and confirm the clinical utility of the MSKCC dynamic risk assessment system in a cohort of patients evaluated and treated outside the United States.