How to discontinue potassium iodide in combined therapy with methimazole for initial treatment of Graves' disease.

The combination of methimazole (MMI) and potassium iodide (KI) is often used to improve the thyroid function more quickly in the initial medical treatment of Graves' disease when the levels of free T4 are ≥5.0 ng/dL. However, deterioration in the levels of free T3 and T4 was observed immediately after the cessation of KI. To avoid deterioration following cessation of KI, we investigated 150 drug-naïve patients with Graves' disease treated with a combination of MMI and KI. Patients administered KI for 3-17 weeks (median, 9 weeks) during the initial period were selected for this study. Levels of free T3 and T4 were determined before treatment, at the time of cessation of KI, and at 2-8 weeks (median, 4 weeks after cessation). In 35 of 150 patients (23.3%), the levels of free T3 and/or free T4 were elevated beyond the upper limit of each reference range after cessation of KI. In a multivariate regression analysis, the levels of free T3 and the ratio of the daily dose of KI (mg) to MMI (mg) at the cessation of KI were significantly lower (p < 0.0001, p = 0.0007) in patients without deterioration than in those with deterioration. The odds ratios were 0.175 (95% confidence interval [CI]: 0.072-0.381) and 0.675 (95% CI: 0.533-0.846), respectively. When the ratio of the dose of KI to MMI at cessation was ≤1.7, and the level of free T3 was ≤3.2 pg/mL, deterioration was avoided in 97.8% of patients.