I was diagnosed with hypothryoidism at 42, and am now 70.
For decades, I was stable on 200mcg pd. In the last couple of years, tests have shown that my thyroxine levels were too high, and my dose has been reduced to 175, then 150 and now to 125. I'm a bit mystified, as I thought once it went wrong, it stayed wrong ifyswim.
By way of background, my mother and maternal aunt were also hypothyroid, and my mother was on the same dose when she died that she started on in her 40s.