Background: A randomized clinical trial was performed to clarify whether continuous use of methimazole (MTZ) during radioiodine (¹³¹I) 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 eight days before ¹³¹I (-MTZ, n = 36) or to continue MTZ until four weeks after ¹³¹I (+MTZ, n = 39). Calculation of the ¹³¹I 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 weeks after therapy no significant change in serum free T4-index was observed in the +MTZ group (109 ± 106 vs. 83 ± 28 nmol/l at baseline; P = 0.26), contrasting an increase in the -MTZ group (180 ± 110 vs. 82 ± 26 nmol/l, P < 0.001). The number of cured patients was 17 (44%) and 22 (61%), in the +MTZ and -MTZ group, respectively (P = 0.17). Cured patients tended to have a lower 24 h thyroid ¹³¹I uptake (50.1 ± 13.8% vs. 56.4 ± 17.1%; P = 0.09). By adjusting for a possible inter-factorial relationship through a regression analysis (variables: randomization, 24 h and 96 h thyroid ¹³¹I uptake, type and duration of disease, age, gender, presence of anti-TPO-antibodies, thyroid volume, dose of MTZ), only the continuous use of MTZ correlated with treatment failure (P = 0.006), while a low 24 h thyroid ¹³¹I uptake predicted a better outcome (P = 0.006).

Conclusion: Continuous use of methimazole hinders an excessive increase of the thyroid hormones during ¹³¹I 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 ¹³¹I uptake.