Abstract: ¹³¹I imaging is a specific method for demonstrating metastatic differentiated thyroid carcinoma. It was reported that its specificity and sensitivity were 96% and 80% respectively, but false positive results also existed: many tissues besides thyroid can accumulate ¹³¹I such as frontal sinus mucocele, thymus, lung carcinoma, breast, esophagus benign stricture, colon graft, gastric pull-up, renal cyst etc. Although its incidence is low,it often misleads the diagnosis and brings unnecessary radiation to the patients. So it is necessary to make sure the location, cause and possible differentiate way of false positive results. In order to get correct diagnosis results, it is suggested to use other imaging modality or ²⁰¹Tl imaging together and detect serum h-Tg at the same time.