I've been on synthroid for about 15 years.. I had Graves Disease and they nuked my thyroid. Seriously couldn't tell much of a difference. Metabolism is supposed to slow down some. Not sure if it's the synthroid, now other meds due to heart valve replacement 6 years ago, being in better shape or just getting older, but I do eat a lot less and healthier now and weight comes off more slowly. My endurance is pretty good and can do just about anything physically that I want to do.
Good luck.
Larry