Early changes of thyroid hormone concentrations after 131I therapy in Graves’ patients pretreated or not with methimazole

Abstract

SummaryAim: Despite extensive use of 131I therapy for Graves’ hyperthyroidism the treatment regimen with 131I and antithyroid drugs remain under discussion. In our prospective clinical study we followed acute thyroid hormone changes after 131I in patients not pretreated with methimazole (MMI) and in patients with different MMI pretreatment regimens. Patients, methods: 187 patients were treated with fixed activity of 550 or 740 MBq of 131I. First group (71 patients) received 131I alone. In the second group (57 patients) MMI was stopped seven days before 131I. The third group (59 patients) received MMI until 131I application. Initial free triiodothyronin and free thyroxin were measured in the second group 7 and 2 days before 131I therapy and in all three groups on the day of 131I application as well as 2, 5, 12, and 30 days afterwards. Absorbed dose was measured in each patient. Results: In the non-pretreated group 131I application was followed by a significant decrease of fT4 in 5 days and of fT3 in 2 days, higher reduction was detected in patients with higher baseline values. In MMI pretreated patients significant but clinically irrelevant increase of both thyroid hormones was detected with maximum value 7 days after discontinuation in the second group and 5 days after discontinuation in the third group. Additionally, in patients of the third group absorbed dose of 131I was significantly lower relative to other two groups. We found no correlation between absorbed dose of 131I and thyroid hormone changes. Conclusion: Our study demonstrates that 131I application alone does not result in exacerbation of hyperthyroidism and therefore it may be considered as safe. Additionally, MMI withdrawal causes significant but clinically irrelevant elevation of thyroid hormones.