Serum Tg assay and screening for interferences Serum Tg was assayed in duplicate by a specific high-sensitive IRMA assay (DYNOtest® Tg-plus, BRAHMS Diagnostica GmbH, Berlin, Germany) according to the producer’ instructions. This method provided a sensitivity of 0.05 ng/mL and a functional sensitivity of 0.2 ng/mL [16, 17]. As previously published, preablative offT4 serum Tg values above 4.5 ng/mL and 3.2 ng/mL were considered as positive with respect to PT-WBS and 12 months restaging results, respectively [8]. Otherwise, offT4-Tg values higher than 0.2 ng/ml measured 12 months after thyroid ablation were considered positive for persisting/relapsing disease. [18, 19]. The presence of anti-thyroglobulin antibodies (AbTg) was screened by a specific radioimmunoassay (DYNOtest® anti-TGn, BRAHMS Diagnostica GmbH, Berlin, Germany) and by recovery test with a specific Tg-recovery buffer provided by the producer. Sera showing AbTg levels more than 60 U/mL and/or recover less than 80% or more than 120% were excluded from the study. Quality control was ensured by assaying two levels of control sera in each series, by re-assessing all sera showing a coefficient of variation exceeding 10% and by a bimonthly partecipation in the European inter-laboratory control OncocheckTM.