Objective To assess the value of apparent diffusion coefficient (ADC) in diffusionweighted imaging (DWI),thyroid radioactive iodine uptake (RAIU)and serum parameters in differential diagnosis between Graves disease(GD)and painless thyroiditis(PT).Methods One hundred and two patients with GD and 37 patients with PT were enrolled.Serum thyroid hormones and antibodies were measured,including free triiodothyronine (FT3),free thyroxine (FT4),TSH,thyroid globulin antibody (TgAb),thyroid peroxidase antibody (TPOAb) and thyrotropin receptor antibody (TRAb).DWI was obtained with a 3.0 T MR scanner,and ADC values were calculated.24 h later RAIU and thyroid sc intigraphy were conducted.Tissue samples were obtained in GD patients (6 cases) after thyroidectomy and in PT patients(2 cases)after biopsy.Pearson bivariate correlation was made.Receiver operating characteristic curves (ROC) were drawn and diagnostic efficacies were determined.Optimal cut-off values were selected,and then sensitivity,specificity,accuracy,positive predictive value and negative predictive value were assessed.Results ADC,TRAb and RAIU were significantly higher in GD than in PT (t=15.126,7.226 and 31.574,with all P＜0.01).And they were closely and positively correlated.ROC showed areas under the curves for RAIU,ADC and TRAb were higher than 0.900.RAIU was the best method to differentiate GD from PT.When cutoff value of RAIU was determined as 24.500 ％,sensitivity,specificity,accuracy,positive predictive value and negative predictive value were all 100％.The above statistical data were 96.078％,91.892％,95.000％,97.059％ and 89.474％ for ADC,and 88.235％,75.676％,84.892％,90.909％ and 70.000％ for TRAb,when the optimal thresholds of 1.837×10-3 mm2/s and 1.350 IU/ml were determined respectively.ADC was better than TRAb.Histopathology results showed that for GD,follicle hyperplasia,follicular epithelial cells hyperplasia,hyper-vascularity and congestive perfusion were characteristic features; while for PT,massive lymphocytic infiltration with hyperplastic germinal center formation and follicle disruption were characteristic features.Conclusions RAIU,DWI and TRAb were all of diagnostic values for differentiation between GD and PT,RAIU was the best method and DWI followed.DWI has great potential for thyroid pathophysiological imaging because it reflects tissue cellularity differences between GD and PT.The principle for RAIU is that it reflects the iodine uptake changes in different diseases.