Continuous methimazole therapy and its effect on the cure rate of hyperthyroidism using radioactive iodine:: An evaluation by a randomized trial

Background: A randomized clinical trial was performed to clarify whether continuous use of methimazole (MTZ) during radioiodine (I-131) therapy influences the final outcome of this therapy. Design: Consecutive patients with Graves' disease (n = 30) or a toxic nodular goiter (n = 45) were rendered euthyroid by MTZ and randomized to stop MTZ 8 d before I-131 (-MTZ; n = 36) or to continue MTZ until 4 wk after I-131 (-MTZ; n = 39). Calculation of the I-131 activity included an assessment of the 131I half-life and the thyroid volume. Results: The 24-h thyroid 131I uptake was lower in the +MTZ group than in the +MTZ group (44.8 +/- 15.6% vs. 62.1 +/- 9.9%, respectively; P < 0.001). At 3 wk after therapy, no significant change in serum free T-4 index was observed in the +MTZ group (109 +/- 106 vs. 83 +/- 28 nmol/liter at baseline; P = 0.26), contrasting an increase in the -MTZ group (180 +/- 110 vs. 82 +/- 26 nmol/liter; P < 0.001). The number of cured patients was 17 (44%) and 22 (61%) in the +MTZ and -MTZ groups, respectively (P = 0.17). Cured patients tended to have a lower 24-h thyroid I-131 uptake (50.1 +/- 13.8% vs. 56.4 +/- 17.1%; P = 0.09). By adjusting for a possible interfactorial relationship through a regression analysis (variables: randomization, 24- and 96-h thyroid I-131 uptake, type and duration of disease, age, gender, presence of antithyroid peroxidase antibodies, thyroid volume, dose of MTZ), only the continuous use of MTZ correlated with treatment failure (P = 0.006), whereas a low 24- h thyroid I-131 uptake predicted a better outcome (P = 0.006). Conclusion: Continuous use of MTZ hinders an excessive increase of the thyroid hormones during I-131 therapy of hyperthyroid diseases. However, such a strategy seems to reduce the final cure rate, although this adverse effect paradoxically is attenuated by the concomitant reduction of the thyroid I-131 uptake.