Abstract: There are 52 patients with DTC who were referred to our department for radioiodine ablation for the first time were adopted. 30 patients (experimental group) were given a diagnostic scan (Dscan) 24 hours after the administration of 74 MBq ¹³¹I before radioiodine therapy, and a therapeutic scan (Tscan) was performed after radioiodine therapy. The fractional concentrations of ¹³¹I in remnants or functional metastases were quantified on Dscan and Tscan, and were expressed as D_x and T_x respectively. 22 patients (control group) received ¹³¹I therapy directly after thyroidectomy. And Tscan was performed after the radioiodine ablation. As a result the mean T_x(experimental group) was lower both than D_x(experimental group) and T_x(control group). According to the interval time between the diagnostic dose and therapy dose was 1.2, 3.7 and 832 days, the experimental group patients were divided into three subgroups A、B、C. The mean T_x/D_x of subgroup B was lower than T_x/D_x of subgroups A and C. There was no statistically significant difference of T_x/D_x between subgroup A and C. In conclusions that 74 MBq ¹³¹I for diagnostic scan can cause thyroid stunning. And the stunning effect is more significant when the radioiodine ablation was performed on 3 to 7 days after diagnostic dose.