Yes and no. Your thyroid is responsible for regulating your metabolism. Synthroid is synthetic t4. Once in your body this will convert to t3 at a 4:1 ratio. (so for every 4mg of synthroid/t4 you take, it will convert to 1mg of t3.) taking in synthroid will not slow down your metabolism, but rather speed it up! ... It's a common weight loss drug on par with clenbuterol and ephedrine. Very effective if used properly.
However, prednisone *will* cause weight gain. It will contribute to fluid retention and to increased appetite. There really is not much you can do about this other than schedule your meals and stick to a proper diet. This is a very common complaint with prednisone users and often causes many women to fall into depression. The only thing you could really do is talk to your doctor about taking an alternate medication to be honest. And yes, you are right.. You will likely continue to gain weight up to around 25lb or so. :(