Introduction: Measurement of postoperative stimulated serum thyroglobulin (s-tg) has the potential to predict persistence and recurrence of papillary thyroid carcinoma (PTC).
Aim: The objective of this study is to examine the impact of s-tg on subsequent disease-free status.
Materials and Methods: We included patients submitted to total thyroidectomy for PTC with postoperative s-tg measurement. Statistical analysis was performed using Stata 15. Descriptive statistics were performed for every variable and comparisons between groups used the t-test for continuous and the chi-square for binary variables. Survival analysis was performed by the Cox model and classification analysis using regression trees for definition of thyroglobulin cut point.
Results: We retrospectively analyzed 1,319 consecutive patients with PTC submitted to total thyroidectomy. There were 1,058 females (80.21 %) and 261 males (19.79 %). Age ranged from 9 to 84 years (mean, 43.49 years, SD 13.25 years). Central neck dissection was performed in 241 patients (18.27 %). All patients were submitted to radioiodine remnant ablation. Multifocal disease was present in 469 patients (35.56 %) and metastatic lymph nodes were diagnosed in 223 patients (6.91 %). Extrathyroidal extension was diagnosed in 264 patients (20.02 %). Stimulated thyroglobulin level ranged from values below 0.10 to 14,400 mg/dL. Due to its distribution format, a logarithmic transformation was used for data normalization. Time of follow-up ranged from 0.73 to 236.48 months (mean, 74.30 months, SD 42.78 months). Recurrence was diagnosed in 61 patients (4.62 %) with 53 cases of nodal recurrence, 6 cases of distant metastasis and two cases of synchronous distant and nodal recurrence. Stimulated thyroglobulin was a significant predictor of recurrence-free survival (HR: 1.323, 95 % CI: 1.202 - 1.445, p<0.001). In our series, a cut-off value of 18 was identified as the best for variable dichotomization. Patients with stimulated thyroglobulin above 18 had an HR of 11.148 (95 % CI: 6.278 - 18.470, p<0.001) compared to those below this level. Using a ROC curve approach, the cut-point is pushed back to 2.999. Using this cut-point, a HR significant difference is observed between the two groups (HR: 6.444, 95 % CI: 3.269 - 12.704, p<0.001).
Conclusions: Stimulated tg testing is a readily available tool with a high negative predictive value for future disease-free status. A low s-tg should be considered a favorable risk factor in patients with PTC.