I have been on Synthroid since 2003, I had a total thyroidectamy(thyroid cancer). I have to say anytime I have gained weight on synthroid has been my own fault! Giving into sweets or that extra drink with dinner.
It didn't take long for them to regulate my dosage, they slowly increased it until we found the sweet spot. And then every 3-6 months for the first few years I had blood work done to make sure it was ok. I am now on blood work once a year and the dosage is pretty stabalized.

I will also say that I do now tend to put the weight on in my middle, as if I was older then I am. But... I am also still a size 2/4 so I can't complain to much! I just need to get motivated to do crunches on a regular basis!

So I am not sure why people say these "horror"stories. It is a synthetic hormone to replace what your body is not producing. Not sure how that could be any worse then what you have at the moment!!