Version July 2025
131-I: iodine 131; fT4: free thyroxine; MMI: methimazole; RAI: radioactive iodine; T3: triidothyronine; TSH: thyroid-stimulating hormone; WBC: white blood cell count.
* When treating with MMI, initially monitor TSH, fT4, and T3 every 4 to 6 weeks and adjust dose of MMI to normalize these values. Once these values are in the normal range, continue to monitor fT4, T3, and TSH every 3 to 4 months. Once TSH has recovered fully from suppression, TSH alone may be monitored. During any febrile illness, stop MMI and check WBC with differential to monitor for the development of granulocytopenia.
¶ Major adverse effects of MMI include agranulocytosis, vasculitis (lupus-like syndrome), or hepatitis. Minor adverse effects include rash, hives, arthralgias, transient granulocytopenia, or gastrointestinal symptoms.
Δ Practice varies regarding the length of time that a provider will continue to treat with MMI after treatment has resulted in control of the hyperthyroidism. We do not recommend a trial off of MMI after an arbitrary period of treatment. Instead, we prescribe MMI indefinitely, providing that the antithyroid drug remains the preferred treatment options of the patient and parents and that no major adverse effects occur. Refer to UpToDate topic text for details.
◊ Patient should be off of MMI for 3 to 5 days before RAI treatment. We suggest using a RAI dose calculated to achieve hypothyroidism.