The dosage by weight system for Levothyroxine is a very rough starter rule of thumb, and a bit like starting people on rapid action insulin being advise to carb count at 1 unit bolus per 10gr of carb. Like insulin doses, actual requirements can be very different.
Based on my weight, my dose should be c80 mcgr per day. On that, my thyroid performance was still degrading. 150mcgr still didn't stem the decline.
I am now on 125mcgr Levothyroxine, plus 50 mcgr Liothyronine per day and just about holding steady. It has taken since March 2018 to reach this point.
Many, many, MANY of those living with hypothyroidism live their lives under medicate.