Objective To investigate the influential factors for the outcome of the first 131I therapy in patients with PTC after total thyroidectomy.Methods One hundred and fifty-nine patients (45 males,114 females,average age (43.4± 12.2) years) with PTC after total thyroidectomy who underwent 131I therapy from July 2014 to December 2015 were retrospectively analyzed.Curative efficacy was evaluated 6 months after 131I therapy.Therapeutic outcome was evaluated according to TSH stimulated Tg (sTg) level,Dx-WBS and evidences of other imaging modalities.Twelve possible factors affecting therapeutic outcome of 131I therapy,including patients' age,gender,time interval between thyroidectomy and 131I therapy,primary tumors size and extrathyroidal extension,number and range of primary tumor lesions,lymph node metastases in surgery,status of thyroid remnant in 99TcmO4-imaging,pre-treatment laboratory measurements (TSH,sTg and TgAb),131I therapeutic dose,results of Rx-WBS and SPECT after 131I therapy,were analyzed with univariate and multivariate logistic regression.ROC curve and diagnostic critical point were analyzed to evaluate the predictive value of influential factors for the outcome of 131I therapy.Results The cure rate of the first 131I therapy was 64.2％ (102/159).Univariate logistic regression analysis showed that age,gender,lymph node metastases,sTg and 131I therapeutic dose (all P＜0.01) were the influential factors for the outcome of 131I therapy.Multivariate logistic regression analysis showed that lymph node metastases (regression coefficient:1.118) and sTg (regression coefficient:0.314) were influential factors (both P＜0.05).The regression equation was:Logit P =-4.155+ 1.118×lymph node metastases+0.314×sTg (x2 =93.7,P＜0.001).Taking sTg as a predictive factor for the outcome of 131I therapy,the AUC of ROC curve was 0.926 (95％ CI:0.888-0.963).The cut-off value of sTg was 2.97 μg/L with a sensitivity of 94.7％ (54/57) and a specificity of 76.5％ (78/102).Conclusions PTC patients with low sTg levels and few lymph node metastases after total thyroidectomy are more likely to be cured in the first 131I therapy.