Hello – It used to be that docs would insist on stopping anti-thyroid drug therapy after 18-24 months, but the latest guidance from the American Thyroid Association and American Association of Clinical Endocrinologists (which is located in the “Treatment Options” thread in the announcements section of the forum) actually does leave the door open for longer-term use if that is the patient’s preference. With PTU, there is a specific issue that increases in risk the longer you are on the meds (ANCA-positive vasculitis), but I’m not sure if this is an issue with methimazole.
This piece from the American Thyroid Association’s 2017 meeting might be of interest, although most of the patients had reduced to around 5 mg/day