Questions about IUD

I've been considering changing my bc plan. I've been on the pill for 4 years and really haven't had a problem. I was talking with a friend recently and she was telling me how she was having a hard time losing weight while being on the pill, I agreed. She told me her nutritionalist friend told her the pill makes you retain water weight and can make it difficult to lose weight. So I've been thinking of changing to an IUD but I'm not really familiar with this form of bc. Any help or comments are appreciate!