Patients treated with 131 I may be identified at security checkpoints at various public facilities. The objective of this survey was to determine the frequency of detection, the spectrum of public facilities, the various methods of management of the situation by security agents, and the spectrum of physician documentation for patients regarding their 131 I therapy. Methods: Data were tabulated from a Thyroid Cancer Survivors' Association, Inc., survey emailed to approximately 15,000 associates and available online from December 2013 to December 2014. Responses were tabulated from respondents who reported that they were 18 y old or older, had received at least 1 131 I treatment for differentiated thyroid cancer, and were responding regarding their last 131 I treatment. Results: Of 621 respondents, 595 reported an attempt to pass through a public facility security checkpoint. Of these 595 patients, approximately 10% (57) were identified as being radioactive. The facility reported by 43 respondents was an airport for 35% (15), border crossing for 33% (14), government building for 19% (8), shopping mall for 7% (3), train station for 5% (2), and steel recycling plant for 2% (1). The security agent's management of the situation reported by 47 respondents included questioning for 81% (38), allowing them to proceed without a change in travel plans for 57% (27), requesting documentation of the therapy for 55% (26), rescanning for 55% (26), calling a member of the treating team for validation for 17% (8), "strip" searching for 4% (2), detaining such that a change in travel plans was required for 6% (3), and prohibiting continued travel for 4% (2). The period of detainment reported by these 47 respondents was less than 30 min for 57% (27), 30 to less than 60 min for 21% (10), 1 to less than 1.5 h for 15% (7), 1.5 to less than 2 h for 2% (1), 2-4 h for 0% (0), and greater than 4 h for 4% (2). Data regarding physician documentation are presented. Conclusion: The detection of radioactivity at a variety of security checkpoints at public facilities after 131 I therapy occurred in approximately 10% of respondents. Travel inconvenience is not infrequent and may require alteration of travel plans. Physicians should take steps to ensure that patients not only have appropriate documentation of their 131 I therapy with them but also have instructions regarding how security agents may verify their 131 I therapy.