Long-term methimazole therapy in Graves' hyperthyroidism and adverse reactions: a Danish multicenter study

Purpose: In this prospective multicenter study with patients newly diagnosed with Graves' hyperthyroidism (GH), we studied the timing and characteristics of adverse drug reactions in patients treated with anti-thyroid drugs (ATD) for up to 48 months. Methods: Patients with GH were treated with ATD until remission and hereafter with a low-dose regime to keep the patients in remission. The patients were followed with blood samples and recording of adverse events approximately every second month for the first 2 years and every third month for the following 2 years. Results: We included 208 patients and the patients were treated for a median of 22 (range: 0.5-49) months. Ten percent of the patients experienced adverse drug reactions and 75% of the cases occurred during the first 6 months. After 24 months, the methimazole dose was lowered to 5 mg/day, and after this time point, no further adverse drug reactions were recorded. Skin reactions were the most prominent reaction, comprising 68% of the registered reactions, and no hepatic and bonemarrow affection was recorded. Conclusion: With this study, we report the frequency, timing of occurrence, and characteristics of adverse drug reactions when treating GH with the ATD drug methimazole for up to 48 months. Long-term low-dose methimazole treatment can be a cost-effective and straightforward treatment option if adverse drug reactions such as severe hepatic and bone marrow affection are kept in mind.

Automated summary

This study aimed to investigate the timing and characteristics of adverse drug reactions in patients with Graves' hyperthyroidism (GH) treated with anti-thyroid drugs (ATD) for up to 48 months. The results showed that 10% of the patients experienced adverse drug reactions, with 75% of the cases occurring within the first 6 months of treatment. Skin reactions were the most common adverse reaction, comprising 68% of the registered reactions. After 24 months, no further adverse drug reactions were recorded when the methimazole dose was lowered to 5 mg/day.