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Background: Serum thyroglobulin (Tg) is essential to manage differentiated thyroid carcinoma (DTC). However, Tg determination is affected by circulating Tg antibodies (TgAb), and a role of TgAb as surrogate biomarker has been proposed. Here we evaluated the role of TgAb measured before and after radioiodine ablation (RRA) as potential predictors of prognosis., Methods: Patients treated since 2006 were screened. Cancers with structural relapse were defined as recurrent. Both Tg and TgAb were measured by immunoassays on the fully automated Kryptor® platform (BRAHMS Gmbh, Henningsdorf, Germany)., Results: A series of 215 DTC patients was enrolled, of whom 28.8% had positive preablation TgAb. Overall, 2.8% patients died by DTC and 11% recurred. High-risk class (p=0.004) and cancer relapse (p=0.007) occurred more frequently in positive TgAb, whereas better disease-free survival was observed in negative group (hazard ratio 2.59, p=0.01). Having positive preablation TgAb was significantly associated with risk to develop recurrence (odds ratio 3.57, p=0.004). Among positive TgAb subgroup, higher levels were recorded in recurrent cases (p=0.0001), and the most accurate preablation TgAb threshold was 107.5 IU/mL. When TgAb were measured at first follow-up, recurrence rate was significantly (p<0.0001) higher in persistently TgAb-positive patients (75%) than normalized ones (2.4%). At that time, the highest negative predictive value could be obtained when considering TgAb normalization (<33 IU/mL) or reduction by ≥36.4%., Conclusions: Positive TgAb before RRA indicates higher risk of poor prognosis, but their significant drop 6-12 months later could be considered a favorable factor.