Is three-day withdrawal of methimazole appropriate in radioiodine therapy in Graves' hyperthyroidism?

Objectives To evaluate whether 3-day withdrawal of methimazole (MMI) prior to radioactive iodine therapy (RAI) is adequate in Graves' disease with adjunctive MMI treatment.

Methods Thirty-eight patients with Graves' hyperthyroidism (M15/F23. age 21-78yrs. average of thyroid weight 68g) who have been treated by MMI for 1 to 6 years were enrolled. Low iodine diet aas applied from 7 days before RAI to 4 days after RAI. Administered dose of I-131 was individually adjusted so that absorbed dose in the thyroid may exceed 150 Gy. MMI was stopped 3 days prior to RAI. Serum levels of FT3, FT4, TSH and TRAb as well as 24hr uptake of radioiodine were determined on the day of RAI. Therapeutic outcome was judged at 6 months after RAI. Achievement of hypothyroidism was considered as successful. The results of laboratory tests, uptake, and outcome were compared to those in 30 patients undertook RAI with conventional protocol of 7-day withdrawal of MMI.

Results Serum levels of FT3 (pg/ml) and FT4 (ng/dl) in 3-day group were significantly lower than those in 7-day group (8.0±4.5 vs.13.3±4.7,p<0.01, and 3.1± 2.2 vs. 4.9±2.8, p<0.01). There were no statistaical difference in 24hr uptake between 3-day group vs. 7-day group (72.8±9.4 vs. 76.3±12.8). Success rate of RAI in the 3-day group was comparable to that in 7-day group (89% vs. 90%,p=ns)

Conclusions RAI with 3-day withdrawal of MMI is appropriate and effective in treating Graves' disease without exacerbating hyperthyroidism.