Radio-iodine is a safe and effective treatment of thyrotoxicosis, but carries a risk of the late development of hypothyroidism several years after therapy. Patients treated with radio-iodine require long-term follow-up to ensure that late hypothyroidism is detected and treated. Current changes in health care commissioning propose an increasing reliance on primary care for the long term follow-up of patients with thyroid disease. We report a study of the 8 year follow-up of 100 consecutive patients treated with radio-iodine for hyperthyroidism (mean dose 402 MBq) between January 1999 and March 2000. All patients had been instructed to ensure that thyroid function was tested at least annually following therapy. Data on thyroid function testing during a recent 18 month period (January 2007June 2008) and information about thyroxine replacement therapy were gathered from local hospital laboratories and general practice records. For 41 patients there was no evidence of a recent thyroid function test during the 18 month study period. Further enquiry revealed that 15 had died since therapy; of the remaining 26 patients, 15 were still registered locally, 5 were known to have moved away and 6 were untraceable. Of the 59 patients for whom recent information was available, 50 (85%) were taking thyroxine replacement therapy; for 18 of these (36%) the last TSH result was outside the reference range (0.274.2 mU/l), 7 having a raised TSH and 11 a low TSH (of which 6 were <0.1 mU/l). Those without evidence of recent follow-up included patients known to have abnormal thyroid function when last tested. These results demonstrate that current arrangements for long tem follow-up of patients treated with radio-iodine are not secure. It is likely that a thyroid register would improve follow-up and reduce the risk of undiagnosed iatrogenic hypothyroidism. Commissioners of modern health-care processes should be aware of this need.