Context Despite its good prognosis, differentiated thyroid cancer (DTC) is characterized by high rates of disease persistence and recurrence. Estimation of long-term remission (excellent response) thanks to specific parameters could help to individualize the active surveillance schedule. Objective Evaluation of the ability of stimulated thyroglobulin (Tg) and Tg reduction index (TRI) to predict long-term remission in patients with DTC managed by thyroidectomy and radioactive iodine (RAI) remnant ablation. Patients and Interventions Observational retrospective study of 1093 patients treated for DTC between 1995 and 2010. Preablation stimulated thyroglobulin (presTg) was measured under thyroid hormone withdrawal just before RAI. Recombinant human TSH–stimulated thyroglobulin (sTg) was measured at first evaluation of the initial management 6 to 12 months after RAI. TRI was calculated based on pre-Tg and sTg. Results After univariate and multivariate analyses, lymph node invasion (N1, OR = 2.08; 95% CI, 1.19 to 3.64), presTg (OR = 4.04; 95% CI, 2.56 to 6.38), sTg (OR = 2.62; 95% CI, 2.05 to 3.34), and TRI (OR = 0.43; 95% CI, 0.21 to 0.88) were identified as independent prognostic factors influencing the rate of disease persistence or recurrence after the initial management. Receiver operating characteristic analysis identified presTg cutoff (<10 µg/L) to predict excellent response, with a negative predictive value of 94%, and validated for higher stages (T3/T4, N1). Furthermore, sTg <1 µg/L predicts excellent response. TRI >60% for the entire cohort and 62.5% for locally advanced disease (T3/T4, N1) was sensitive predictor for excellent response. Conclusion This study identifies presTg, sTg, and TRI as highly sensitive predictors of excellent response in patients with DTC and subsequently disease-free status. The cutoff of such parameters is also adapted for patients with higher tumor stages (T3/T4, N1).