Predictive value of serum thyroglobulin after surgery and prior to I-131 therapy in patients with differentiated thyroid carcinoma, examined at two different time intervals

Background: Differentiated thyroid carcinoma (DTC) comprises nearly 90% of all thyroid malignancies. The main focus of our study was to examine how measuring Tg at multiple time points could predict the persistence and/or recurrence of both local and distant diseases. Methods: In this retrospective, single-center, observational study, we examined records of all patients who underwent total thyroidectomy for DTC and received radioactive iodine (RAI) ablation/therapy at the Nuclear Medicine of Shahid Beheshti Hospital. Blood samples were collected at two time, approximately 30 days before RAI (Tg-30 and TSH-30) and the same day of RAI, after TSH-stimulation (Tg-0 and TSH-0). During the follow-up period, patients were categorized into the four groups according to the ATA guideline. Group with an excellent response (ER), nodal disease (ND), distant disease (DD) and partial response (PR). Results: A significant difference between Tg-0 and Tg-30 was observed only in the ER group and the other 3 groups (p<0.001), and no significant difference was observed between the remaining 3 groups (p>0.05). Cut-off point of 1.35 ng/mL for the Tg-30 and 4.7 ng/mL for the Tg-0 was used with best sensitivity and specificity for differentiation of ER vs. ND. A cut-off point of 14.25 ng/mL for the Tg-30 and 40 ng/mL for the Tg-0 was able to correctly identify ER vs. DD. Conclusion: By using the cut-off values obtained in this study, it is possible to more accurately predict local or distant recurrences. Maybe these patients need a shorter follow-up period that leads to more appropriate treatment decisions.