Predictive factors of post-operative thyroglobulin <0.2 ng/ml and pre-ablation stimulated thyroglobulin > 10 ng/ml in differentiated thyroid carcinoma

Introduction: To identify predictive factors of post-operative thyroglobulin (Tg) <0.2 ng/ml and pre-ablation stimulated Tg (s-Tg) > 10 ng/ml in differentiated thyroid carcinoma (DTC) patients.

Methods: Data from DTC patients with post-operative Tg <0.2 ng/ml and negative anti-Tg antibody (TgAb) and received radioiodine (RAI) ablation in our tertiary center between January 2009 and January 2019 were retrospectively analyzed. Independent and non-paired Student's t-test and Mann-Whitney U test were employed for continuous variables. For categorical variables, a Pearson's Chi-square test or Fisher's exact test were performed. The cut-off values of continuous variables were determined using receiver operating characteristic (ROC) curve. Variables in univariate analysis were included in the multivariate logistic regression analysis. All analyses were 2-tailed tests based on &alpha; = 0.05 and R version 3.4.0 were used for data analysis (http://www.R-project.org).

Results: Out of 1152 DTC patient with post-operative Tg <0.2 ng/ml and negative anti-Tg antibody (TgAb), 162 (14.1%) patients' s-Tg were more than 10 ng/ml and 990 (85.9%) patients' s-Tg were no more than 10 ng/ml. The gender, tumor size, lymph node ratio (LNR), stage, ATA risk stratification, hypertension, and drinking history were significantly different between two groups (P<0.05). The best cut-off value of age, tumor size and LNR was 36 years, 19.5mm, and 32.9%, respectively. In multivariate logistic regression, only tumor size >19.5mm (OR=3.2, 95%CI: 1.7-4.6, P<0.01) and LNR >32.9% (OR=4.16, 95%CI: 2.3-7.8, P<0.01) were associated with s-Tg>10 ng/ml. After a median follow-up of 4.7 years, structural recurrence was diagnosed in 19 patients with s-Tg no more than 10 ng/ml and 11 patients >10 ng/ml, respectively. No difference was found between two groups in structural recurrence (p>0.05). Additionally, we compared s-Tg level and structural recurrence according to ATA risk stratification, two hundred, 626 and 245 of 1152 patients were low-, intermediate-, and high-risk. 7.5% (n=15), 12.1% (n=76), and 17.6% (n=43) patients with s-Tg >10 ng/ml in three risk stratification group. Structural recurrence was diagnosed in 13.3% (n=2), 5.3% (n=4), and 11.6% (n=5) patients in three risk stratification groups of patients with s-Tg>10 ng/ml. And structural recurrence was diagnosed in 1.6% (n=3), 1.8% (n=10), and 3.0% (n=6) patients in three risk stratification groups of patients with s-Tg

Conclusions: A large proportion of DTC patients with post-operative Tg <0.2 ng/ml and negative anti-Tg antibody (TgAb) are likely to present with s-Tg > 10 ng/ml, which indicating a poorer outcome. Tumor size >19.5mm and LNR >32.9% were predictors of s-Tg>10 ng/ml in those patients.