Objective: Inorganic iodine is often used to treat patients with Graves thyrotoxicosis who do not tolerate thionamides due to adverse effects. However, predictors of continued inorganic iodine efficacy have not been fully elucidated. This study aimed to investigate the factors affecting the continued efficacy of potassium iodide (KI) in patients with Graves thyrotoxicosis.
Methods: In this study, among 1,197 patients with Graves disease who were initially treated with thionamides, we retrospectively studied 24 consecutive Japanese patients whose treatment was changed to KI alone due to the adverse effects of thionamides. We divided these patients into 2 groups: patients who had maintained euthyroid function for at least 180 days (nonrecurrence group, n = 11), and patients who had not maintained euthyroid function for 180 days (recurrence group, n = 13).
Results: Free triiodothyronine (FT3) and free thyroxine (FT4) levels on the day of changing from thionamides to KI were statistically higher in the recurrence group than in the nonrecurrence group (FT3, 9.3 [range, 5.2-11.6] vs. 3.7 [3.3-4.8] pg/mL, P = .02 and FT4, 3.6 [1.8-4.5] vs. 1.4 [1.2-1.9] ng/dL, P = .02). FT4 levels on the day of drug change were significantly higher in the recurrence group, even after adjusting for thionamide or KI dose. In the recurrence group, the duration of KI effect was inversed correlated with FT3 and FT4 levels on the day of drug change.
Conclusion: Continued efficacy of KI after thionamides might be inversely correlated with thyrotoxicosis severity on the day of drug change.
Abbreviations: ANOVA = analysis of variance eTV = estimated thyroid volume FT3 = free triiodothyronine FT4 = free thyroxine IQR = interquartile range KI = potassium iodide MMI = thiamazole PTU = propylthiouracil RAIT = radioactive iodine therapy TRAb = TSH receptor antibody TSH = thyroid stimulating hormone.