I'm 41 but have a similar history to you but with a different outcome. I was diagnosed at 26 following the birth of my son as being hypothyroid. I had no symptoms but the doc felt my neck during a routine exam and ordered bloodwork. I've been on 100mcg of Levoxyl since then and have had no problems with weight, actually I'm the opposite, I've always been underweight. I've since had surgery for cancer and am now playing the "find the right dosage" game. My point is, maybe it's the meds? I'm no doctor so I don't know but if you are exercising and eating right then maybe ask your doc if it could be the synthroid.
You sound very similar to me. From what I hear, once you have a thyroid problem you can struggle with your weight for the res of your life. Like you I exercise and eat pretty good and just can't seem to loose. It gets very frusterating but think about what could happen if you don't keep up the exercise. This is what keeps me on the tredmill even when I really don't feel like it. Best of luck to you.
Janelle
Sorry -- some background on me 24f with 2 kids -- thyroid problems started about 6m postpardom with last child or about 1 yr ago.