Abstract： Objective:To investigate the value of cellular immune status before initial 131I treatment for predicting treatment response in young and middle-aged patients with papillary thyroid cancer (PTC). Methods:From March 2018 to April 2019, 150 young and middle-aged patients with PTC (46 males, 104 females, age (40.0±9.8) years) who underwent total thyroidectomy and neck lymph node dissection in the Affiliated Hospital of Qingdao University were enrolled retrospectively. All patients underwent radioablation 1-2 months after operation, and the serum lymphocyte subsets (CD3 + , CD4 + , CD8 + , CD4/CD8) as well as natural killer (NK) cells were detected 1 d before the initial 131I treatment. Patients were divided into excellent response (ER) group and non-ER group according to the response of 6-12 months after 131I treatment. Clinicopathological characteristics, preablative stimulated thyroglobulin (psTg), initial 131I dose and lymphocyte subsets that might affect the response to 131I treatment were analyzed (independent-sample t test, Mann-Whitney U test, χ2 test, multiple logistic regression analysis). ROC curve analysis was used to evaluate the predictive value of significant factors for non-ER. Results:Of 150 patients, 84 cases were in ER group (56.00%), and 66 cases (44.00%) were in non-ER group. Age ( z=-2.86, P=0.004), M stage ( χ2=13.64, P<0.001), psTg ( z=-8.94, P<0.001), initial 131I dose ( z=-7.60, P<0.001), CD4 + ( t=2.50, P=0.014), CD4/CD8 ( z=-2.22, P=0.027) of the two groups were significantly different. Multivariate analysis showed that psTg (odds ratio ( OR)=1.27, 95% CI: 1.16-1.40, P<0.001) and CD4/CD8 ( OR=0.39, 95% CI: 0.15-0.99, P=0.048) were independent factors for predicting 131I treatment response. The cut-off values of psTg and CD4/CD8 for predicting non-ER were 6.78 μg/L and 1.67, respectively. Conclusions:Cellular immune status before initial 131I treatment may predict treatment response in young and middle-aged patients with PTC. It indicates non-ER response when Tg is higher than 6.78 μg/L and CD4/CD8 is lower than 1.67.