Thyroglobulin autoantibodies before radioiodine ablation predict differentiated thyroid cancer outcome

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 (R) 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.