External dose rate (ED) and residual activity (RA) of patients treated with (131)I sodium iodide are two main factors, to consider before release of patients. In this study, six nuclear medicine centres out of total seven centres in Iran were selected and measurements have been done on 330 patients. ED of patients was measured by physicists of the centres for 6 months (May-November 2009) at a 1 m distance from the thyroid of each patient on the first, second and third days after administration by a calibrated survey meter. The maximum and minimum values of ED were 21 (SD = 18) and 11 (SD = 4) µSv h(-1), respectively. Furthermore, the maximum and minimum values of RA during release of patient were 720 and 250 MBq, respectively. According to the study, we recommend a release activity limit of 500 MBq (14 mCi) or a dose rate level of 20 µSv h(-1) at 1 m from the patient to be set instead of a release activity limit of 1100 MBq (30 mCi) as it is now for the country.