Hypo- Levothyroxine & weight loss?

I went to my doctor for a regular physical last week, and had some blood work done and my thyroid has always been on the high-normal side, and finally the tests from last week came back abnormal. I've been heavy my entire life and recently have gained another 45 lbs over the course of about a year or so. I'm 21 years old and my doctor put me on 25 mcg of levothyroxine. She told me that I will experience some weight loss but I want to know if its going to be significant or just like a few pounds, and how does it work? This is supposed to FIX my metabolism right? So does that mean since I barely eat anything as it is, that I will drop weight fast? In depth answers would be appreciated. I have had a weight problem my entire life and now there is finally a glimmer of hope, but I've read so many articles that are telling me that I'm going to gain... HELP!