Hi Dagmar,
PTU has been used for more than 70 years, and liver problems are reported to exclusively occur when inappropriately high doses are used especially in children. Your doctor must be aware of the reports but have lots of experience using PTU since he's using a conservative starting dose of 200 mg daily. Many doctors still use the old recommendation of 300 mg, and some doctors prescribe very high doses in excess of 600 mg.
Your doctor has also ordered liver function tests, which is good. I'd personally use the PTU and expect to have your dose reduced as soon as FT4 falls into the reference range regardless of the TSH level. This usually takes 6-8 weeks. After your next labs at 2 months, you should check to see if your dose can be reduced. The usual maintenance dose after 8 weeks is 50-150 mg daily.
Both PTU and MMI can rarely cause agranulocytosis, a decrease in the segmented neutrophilic white blood cells. There used to be no treatment available for this condition if it developed but now colony stimulating factors are given should it occur. Like the liver problems, it primarily occurs when inappropriately high doses are used.
Then again, if you notice anything at all that you don't like about PTU or object to its use, you have a right to ask about using methimazole. A 20 mg mmi dose is equivalent to a 200 mg PTU dose.
Best, Elaine