As a woman with an IUD I think it is really important to clear up misconceptions about IUDs. They are a GREAT method of birth control and the most effective method (except fot sterilization).
The amount of hormone released by Mirena is VERY small in comparison to that of birth control pills. It is HIGHLY unlikely that it is the reason for weight gain. Women's bodies are constantly changing and lifestyle is the #1 reason for weight gain. Age, pregnancies, diet and exercise are the major contributors to weight. Unfortunately, I think many women simply use the IUD as an excuse or convince themselves (and others!) into thinking "my IUD is the reason I have gained weight".
Keep a food journal and honestly track everything you eat. This includes bites of your kid's mac n' cheese and the cream you put in your coffee...EVERYTHING adds up! Diet is 80% of weight loss, working out is 20%...what matters most is what and how much you are eating!
I am 5'6" and weight 110 lbs with a Mirena IUD. I am active most days of the week and watch what I eat, but I don't work out like crazy and I drink wine and eat dessert regularly. I have many friends with a Mirena IUD and while there are perks (no periods!) and negatives (cramping or spotting), but NONE can honestly say "Mirena is why I've gained weight".
VERY FEW women many experience some bloating or puffiness around their stomach after having their Mirena inserted. This, however, typically GOES AWAY after a month or so, when your body gets used to it. Keep in mind, it is a foreign object in your body. Everyone reacts differently. But, dramatic LONG TERM weight gain and bloating is NOT from your Mirena. Talk to your doctor if this is happening to you, because you may have an infection or need help with evaluating your diet.
This is my honest experience and I hope it helps some of you!