Prognostic factors for postoperative papillary thyroid cancer with unexplained elevated Tg: A retrospective study

Objective: This study aimed to investigate the underlying reasons for unexplained elevated thyroglobulin (Tg) in postoperative papillary thyroid cancer (PTC) patients according to follow-up results post RAT and to explore the long-term clinical outcomes and prognostic factors associated with these patients. Methods: From April 2016 to June 2019, a retrospective study was conducted on postoperative PTC patients who underwent RAT at our institution. Patients with preablative stimulated thyroglobulin (psTg) > 10 ng/mL but no structurally evident disease were enrolled. The causal categorization for elevated Tg was analyzed 6 months post RAT and the long-term therapeutic responses were assessed at the end of follow-up. To identify risk factors influencing recurrence-free survival (RFS), both univariate and multivariate Cox regression analysis were employed. Kaplan-Meier method was utilized for plotting survival curves. Results: A cohort of 165 subjects was enrolled for the analyses. Based on the results of a six-month follow-up, the postoperative unexplained elevated Tg among 165 patients could be ultimately attributed to thyroid remnant in 13.94% (23/165), biochemical disease in 60.00% (99/165), and structural disease in 26.06% (43/165). With a median follow-up of 58 months, 51 (30.91%), 34 (20.60%), 21 (12.73%), and 59 (35.76%) of the 165 patients achieved ER, IDR, BIR and SIR, respectively. Univariate analysis showed that N stage, TNM stage and suppressed Tg 6 months post RAT may be prognostic factors affecting RFS. Multivariate analysis showed that N1b stage [HR:2.749, P = 0.003] and II/III stage [HR:2.910, P = 0.001] were independent risk factors for RFS. Conclusion: The proportion of 165 postoperative PTC patients with unexplained elevated Tg developing structural disease within nearly 5 years was over 30%. Patients with N1b stage and higher TNM stage were more likely to develop structural disease.