Objective To investigate the efficacy of ¹³¹I therapy and its influencing factors in patients with differentiated thyroid carcinoma (DTC) with a pre-ablation stimulated thyroglobulin (ps-Tg) level of <10 ng/ml.

Methods A retrospective cohort study was conducted to collect the clinical, imaging, and histopathological examination data of 190 intermediate-to-high risk DTC patients who underwent ¹³¹I therapy at the First Hospital of Shanxi Medical University from January 2021 to December 2022. The patients included 61 males and 129 females, with the age of (44.5±10.4) years. All patients had undergone total or near-total thyroidectomy and had a ps-Tg level of <10 ng/ml (which were assessed as having an excellent response (ER) or an indeterminate response (IDR) prior to ¹³¹I therapy). All patients had undergone ⁹⁹Tc^m\mathrmO_4^- thyroid imaging prior to ¹³¹I therapy, and ¹³¹I post-therapy whole-body scan (Rx-WBS) was performed 5–7 days after oral administration of the therapeutic dose of ¹³¹I. Based on the dynamic response evaluation system, patients in the post-operative excellent response (po-ER) and post-operative indeterminate response (po-IDR) groups were further divided into the ¹³¹I post-therapeutical excellent response (pt-ER) and ¹³¹I post-therapeutical non-excellent response (pt-nER) subgroups, respectively. The correlation between ⁹⁹Tc^m\mathrmO_4^- thyroid imaging and ¹³¹I Rx-WBS score was assessed through Spearman correlation analysis. Mann-Whitney U rank sum test, t-test, Fisher exact probability method, or χ² test was performed to analyze the differences in measurement and count data between the pt-ER and pt-nER subgroups within each of the two main groups. Indicators with statistically significant differences were included in binary Logistic regression analysis, and the receiver operating characteristic (ROC) curve was used to determine the optimal cut-off value for predicting therapy efficacy.

Results A total of 136 patients belonged to the po-ER group (131 in the pt-ER subgroup and 5 in the pt-nER subgroup) and 54 patients in the po-IDR group (44 in the pt-ER subgroup and 10 in the pt-nER subgroup). A statistically significant difference was observed in the final follow-up efficacy between the po-ER and po-IDR groups (χ²=11.710, P=0.0006). ⁹⁹Tc^m\mathrmO_4^- thyroid imaging showed a statistically difference between the pt-ER and pt-nER subgroups within the po-IDR group (Fisher exact probability method, P=0.0004), and the ¹³¹I Rx-WBS score presented a statistically significant difference between the pt-ER and pt-nER subgroups within the po-ER group (Fisher exact probability method, P<0.05). Within the po-ER group, a mild correlation was observed between ⁹⁹Tc^m\mathrmO_4^- thyroid imaging and the ¹³¹I Rx-WBS score (r=0.328, P=0.0005), and the proportion of patients with an ¹³¹I Rx-WBS score of 3–4 points was higher in the positive group than in the negative group of ⁹⁹Tc^m\mathrmO_4^- imaging positive and ¹³¹I Rx-WBS scores of 3-4 was significantly higher than that of the ⁹⁹Tc^m\mathrmO_4^- thyroid imaging. Within the po-ER group, only T stage displayed a statistically significant difference between the pt-ER and pt-nER subgroups (Fisher exact probability method, P<0.05). Within the po-IDR group, only the ps-Tg level presented a statistically significant difference between the pt-ER and pt-nER subgroups (Z=−2.784, P<0.05), and no statistically significant differences were observed between the two subgroups in terms of age, gender, capsular invasion, tumor multifocality, maximum tumor diameter, N stage, recurrence risk stratification, thyroid-stimulating hormone level, initial therapeutic dose of ¹³¹I, and urinary iodine level (Z=−2.784 to −0.201, t=−0.392–1.272, χ²=0.000–0.188, Fisher exact probability method, all P>0.05). Binary Logistic regression analysis indicated that T stage was not an independent risk factor for the therapeutic efficacy of ¹³¹I therapy in patients within the po-ER group (P>0.05), whereas the ps-Tg level was an independent risk factor for the therapeutic efficacy of ¹³¹I therapy in patients within the po-IDR group (OR=1.596, 95%CI=1.133–2.250, P=0.008). ROC curve analysis revealed that the optimal cut-off value of ps-Tg level for predicting therapy efficacy was 4.95 ng/ml, with a sensitivity of 70%, a specificity of 77%, and an area under the curve of 0.784.

Conclusion DTC patients with a ps-Tg level <10 ng/ml can achieve a favorable therapeutic response after ¹³¹I therapy, and the majority of these patients belonged to the po-ER.