Thyroglobulin-Based Risk Factor Repositioning for Determining Radioactive Iodine Activity in Patients with Papillary Thyroid Carcinoma: a Multicenter Retrospective Cohort Study

We aimed to investigate the impact of various factors including radioactive iodine (RAI) activity on the therapeutic response according to the range of serum thyroglobulin (Tg) in patients with papillary thyroid carcinoma (PTC). A total of 2809 patients were retrospectively enrolled from 24 hospitals. They were divided into four subgroups according to their serum Tg (stimulated Tg, sTg) or anti-Tg antibody (TgAb) levels, measured just before RAI therapy: sTg < 2 ng/mL, 2 ≤ sTg < 10 ng/mL, sTg ≥ 10 ng/mL, and TgAb > 100 IU/mL. The clinicopathologic factors for therapeutic responses, which were classified as acceptable response (AR) or non-AR, were compared in each subgroup. Clinical impact of the pN category on therapeutic response was different among subgroups based on sTg levels (subgroups with sTg < 2 ng/mL (P = 0.057), 2 ≤ sTg < 10 ng/mL (P = 0.032), and sTg ≥ 10 ng/mL (P = 0.001)). The pN category was also a significant factor in the subgroup with TgAb > 100 IU/mL (P = 0.006). The pT category was not associated with therapeutic response regardless of the sTg level. High activities of RAI (≥ 3.70 GBq) were associated with favorable therapeutic responses in only the subgroup with sTg ≥ 10 ng/mL (P = 0.044). Risk factors for response prediction could be repositioned based on the serum Tg before RAI therapy. RAI activity should be determined while considering the serum Tg-aided remnant thyroid or malignant tissues as well as conventional factors