I found that as I recovered and my body got bigger and stronger, it needed more thyroxine to function where as before it could function on a lower amount.
I'm still on a low dose of around 50mcg, but it's stable now and has been for years.
I'm not sure on the biology of it, but perhaps it's to do with other areas of the body functioning better and thus your thyroid ideally needs to function more highly too, but because you have hypothyroidism it doesn't meet the bodies need. Hence needing a higher dose.
I dunnu, I'm not a biochemist lol.