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Thiamazole, also known as methimazole, is a medication used to treat hyperthyroidism.[2] This includes Graves disease, toxic multinodular goiter, and thyrotoxic crisis.[2] It is taken by mouth.[2] Full effects may take a few weeks to occur.[3]

Common side effects include itchiness, hair loss, nausea, muscle pain, swelling, and abdominal pain.[2] Severe side effects may include low blood cell counts, liver failure, and vasculitis.[2] Use is not recommended during the first trimester of pregnancy due to the risk of congenital anomalies, but it may be used in the second trimester or third trimester.[4] It may be used during breastfeeding.[4] Those who developed significant side effects may also have problems with propylthiouracil.[2] Thiamazole is a thioamide and works by decreasing the production of thyroid hormones.[2]

Thiamazole was approved for medical use in the United States in 1950.[2] It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system.[5] It is available as a generic medication.[2] It is also available in Europe and Asia.[6] In 2017, it was the 244th most commonly prescribed medication in the United States, with more than one million prescriptions.[7][8]