From my understanding, antithyroid drugs can be taken for years, and unless there is a reaction, there is no absolute reason to stop. Doctors typically suggest RAI or surgery after one or two years of pills if there has been no sign of remission. This is because usually life is simpler on replacement therapy than on antithyroid meds, and because most patients feel well and normal after RAI and on thyroid hormone supplements pills.
If I remember correctly, one PTU tab has 50 mg. So you are on a low dose. At one point I was on 9 a day for a total of 450 mg per day. I was gradually tapered off but was on it about 9 months total. You don't say how long you have been on it. There are some bad side effects, and you should be getting lab work at least once a month. I believe it should be a CBC and a chem profile. I think they really do try to get patients off as quickly as possible. Did you say why you have hyperthyroidism? That would make a difference I think. I had it from being on a cardiac drug called Amiodarone, so if you have Graves that might make it a different plan.
I was on 9 PTU a day. they tasted horrible and ruined my appetite. From what i understand this is the strongest dose. I was on this dose for around 6 months until i had surgery 11 days ago (3rd jan 07). I had been on carbimazole for 5 years but suffered a reaction to them last summer so was unable to take them anymore.