Gained 50 lbs, due to hypothyroid, will taking meds help shed pounds?

IN the past three years, I have put on 50 pounds. I am active. I swim a mile every weekday morning. walk my dog three times a day, haven't changed my good eating habits, and yet, gained all of this weight. then my hair started to fall out. a lot. and it is coarse like straw. also, I am ALWAYS tired and have to force myself to do things. so the doc gave me some bloodwork and said my thryroid was underactive and prescribed synthroid. I've been on it for two weeks and am just as tired. will I ever get my energy back? will this drug help me to loose the weight?