Although 131I therapy for hyperthyroidism has been in use for more than 50 years, there are wide variations in every aspect of its administration to patients. A recent survey of physicists and clinicians in the UK documented variations in workloads, responsibility for prescribing, and selection for treatment. Wide variations also were noted in terminology, radioiodine preparations, and activities and doses administered. There was a wide range of comments and reasons given by clinicians for their choice of regimen. The use of antithyroid drugs and beta-blockers before or after 131I therapy also was variable. As a result of these findings, a multidisciplinary group was convened to produce recommended guidelines for the administration of 131I in the UK. The implementation of these guidelines will require consensus among clinicians and this will require further evaluation.