Hello and welcome - yes, you *can* change from PTU to methimazole. However, you will need to work with your doctor in order to do this, as the dosing is not exactly the same. Your doctor will make his/her best guess as to how to translate the dose and then will have you come back within a few weeks for followup lab testing to see if your dose needs to be tweaked further.
In general, methimazole *is* the preferred drug, except during the first trimester of pregnancy (as PTU has less risk of birth defects) and also during thyroid storm, as PTU can inhibit the conversion of T4 to T3.
The biggest concern with PTU is potential liver issues, which are most likely to occur within the first 90 days of taking the med. There is another *rare* issue that can occur (a specific type of vasculitis) after prolonged use of PTU, so that is something to be aware of as well.
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