Background: Radioactive iodine (RAI) therapy effectively cures hyperthyroidism although a varying proportion of patients need repeat RAI doses. There is no consensus on the timing of redosing for RAI which varies in different centres between 6 weeks and 12 months depending on the perceived time to respond to therapy.
Aim: We have undertaken a retrospective study to assess if after the initial RAI dose of 400 MBq a second RAI dose can be administered earlier than at 6 months, which is the redosing policy at our centre.
Patients and methods: Retrospective assessment of 199 patients for whom 6 month follow-up data was available. Clinical and laboratory data was obtained from patient notes and electronic database. Cure of hyperthyroidism was defined as achieving euthyroidism or hypothyroidism on the basis on FT4 and FT3.
Results: At 6 weeks post-RAI, 70 patients were hyperthyroid, of whom 58 (83%) patients achieved spontaneous cure and 12 (13%) patients remained hyperthyroid requiring a second RAI dose. At the 3 month-stage, 29 patients were hyperthyroid of whom 17 (59%) patients achieved spontaneous cure and 12 (14%) patients required a second RAI dose. For patients who were hyperthyroid at 3 months, a high initial FT4 at diagnosis predicted persistence of hyperthyroidism at 6 months and the need for redosing (74.47±24.65 pmol/l (mean±S.D.) for patients not cured at 6 months vs 40.19±18.92 pmol/l (mean±S.D.) for those cured, P=0.001). Persistence of hyperthyroidism was not correlated to any other clinical or biochemical predictors studied.
Conclusion: Following the administration of 400 MBq RAI, a significant proportion of patients who were hyperthyroid at 6 weeks and 3 months, revert to euthyroidism by the end of 6 months. In view of this an earlier use of second dose cannot be recommended as a routine measure although this may be considered in patients with high FT4 at diagnosis.