Successful Re-administration of Low-dose of Methimazole (MMI) in Graves' Disease Patients Who Experienced Allergic Cutaneous Reactions to MMI at Initial Treatment and Had Received Long-term Propylthiouracil (PTU)

Objective When patients with Graves' disease show severe allergic cutaneous reactions, physicians often suggest that they undergo radioiodine therapy instead of receiving propylthiouracil (PTU), another antithyroid drug, because anti-neutrophil cytoplasmic antibody (ANCA) -related vasculitis can occur with PTU, especially with long-term use. However, some patients refuse radioiodine therapy and chose PTU. Sometimes PTU treatment may be prolonged. Since the frequency of adverse effects of methimazole (MMI) is dose-related, there is a possibility that we can re-administer a low dose without adverse effects to patients well-controlled with PTU who once experienced an allergic reaction to MMI. Methods I prospectively re-administered a low dose of MMI to patients who previously experienced an allergic reaction to MMI at initial treatment. The dose of re-administered MMI ranged from 5 mg twice a week to 5 mg daily. Patients Nine patients with Graves' disease who developed urticaria at initial treatment with MMI and had been treated with PTU for 6 to 21 years were recruited. Results Eight of the 9 patients were successfully controlled with MMI without allergic cutaneous reactions. Only one patient felt itchiness 2 days after switching to MMI. However, skin change was not observed. Conclusion If the patients show allergic cutaneous reactions as a side effect of MMI at the initial treatment for Graves' disease, then there is a strong possibility that such patients can tolerate a low dose of MMI without adverse effects after the disease activity has subsided.