Synthroid and thyroid support

Someone PLEASE help or offer assistance! I have been on 50mcg Synthroid for about 1.5 yrs now and have gained about 25 pounds!! I am a very, very active triathlete, not just your 30 mins on the treadmill kind of girl. How about running 5-6 miles, riding my bike 40-50 miles, and swimming all about 3 times per week!! How can a human being continue to gain weight at this activity level? I am considering just stop taking it, cold turkey. Has anyone tried any of the supplements for thyroid support, or natural meds? If you've got any suggestions, I am open to just about anything at this point!!

I have only been on Synthroid for 2 months, but my appetite is out of control. I read that gaining weight might be a side effect of Synthroid. You would think that you would lose weight with it since it makes your thyroid run more efficiently. My TSH is 4.3, still normal, so I don't guess I really need it yet. I am going off of it and going to the health food store and take Thyroid support.