Objective To investigate the efficacy response and its influencing factors of the first ¹³¹I therapy in patients with papillary thyroid carcinoma (PTC) after surgery.

Methods A retrospective case-control study analysis was conducted on the clinical data of 379 patients with PTC who underwent total thyroidectomy and subsequent first ¹³¹I therapy at Inner Mongolia Autonomous Region People's Hospital from January 2016 to December 2023. The participants comprised 112 males and 267 females, with a mean age of (46.1±11.0) years. Therapeutic efficacy was evaluated 6 months after treatment. According to the Guidelines for radioiodine therapy of differentiated thyroid cancer (2021 edition), all patients were divided into the excellent response (ER) group and the no-excellent response (no-ER) group. Clinical characteristics, including gender, age, maximum tumor diameter, bilateral tumor, multifocality, extrathyroidal extension, T stage, N stage, number of lymph node metastases, lymph node metastasis ratio, recurrence risk stratification, pre-ablation stimulation thyroglobulin (ps-Tg), were collected and compared between the two groups. Univariate analysis was performed using χ² test and Mann-Whitney U test. Variables with statistical significance in univariate analysis were enrolled into multivariate logistic regression analysis to identify independent influencing factors, and the receiver operating characteristic (ROC) curve was used to evaluate its predictive value.

Results A total of 283 patients (74.7%) were in the ER group and 96 patients (25.3%) in the no-ER group. Univariate analysis showed statistically significant differences between the two groups in terms of gender (χ²=23.259, P<0.001), maximum tumor diameter (χ²=18.754, P<0.001), recurrence risk stratification (χ²=75.384, P<0.001), T stage (χ²=20.220, P<0.001), N stage (χ²=50.168, P<0.001), number of lymph node metastases (χ²=38.602, P<0.001), lymph node metastasis ratio (Z=−4.505, P<0.001), and ps-Tg level (Z=−11.949, P<0.001). Multivariate logistic regression analysis demonstrated that gender (OR=2.588, 95%CI: 1.159–5.780, P=0.020) and ps-Tg level (OR=1.160, 95%CI: 1.103–1.221, P<0.001) were independent influencing factors for ER. ROC curve analysis revealed that the optimal cut-off value of ps-Tg for predicting ER was 10.25 μg/L, with an area under the curve of 0.908 (95%CI: 0.867–0.948) and corresponding sensitivity of 79.2% and specificity of 93.3%.

Conclusions Gender and ps-Tg level are independent influencing factors for achieving ER after the first ¹³¹I therapy in patients with PTC. Patients with PTC showing high-risk clinicopathological features (maximum tumor diameter >1 cm, high lymph node metastasis ratio, large number of lymph node metastases, high recurrence risk, male gender, and ps-Tg>10.25 μg/L) are likely to have no-ER after treatment and thus require intensified follow-up and timely adjustment of therapeutic regimens.