It's helpful to know that you felt better (great, even) on (presumably) Levoxyl
) 225mcg several years ago.
I'm curious what your TSH was back then and also what it is now. You've mentioned that it's normal but if you look at the reference range, you'll see that there's a 10x difference form the lower limit of normal to the upper limit of normal in most labs.
So if your TSH was previously 0.4miu/l a few years ago on 225mcg and now is 4.5miu/l on 150mcg, then you could be considered normal in both instances. However, chances are that you'd wouldn't feel as good at the 150mcg dose than as on the 225mcg dose as you've reported. I'd be curious as to why your dose was dropped.
There's a lot of controversy regarding what TSH is normal, even more so regarding optimal, within the reference range. That's kind of like saying an a grade is passing just like a d grade. Yes, both are passing grades but there's a world of difference between the two. However, i'm not sure that putting you back on 225mcg would fix your shortness of breath. Good luck!