Without seeing your lab results to judge along with your symptoms, I can't say for sure, but I CAN venture an educated guess...
What often happens is that your thyroid gland was still somewhat functional. As you take the Synthroid to lower your TSH (TSH tells the thyroid gland to produce more hormones), your own thyroid gland produces less, and sometimes the result is that you are actually receiving less thyroid hormones now than what you were producing before going on Synthroid.
In essence, you are under-medicated and need a dose increase. I belive you that you lead a healthy lifestyle and have no doubt that you are trying to lose weight and/or keep from gaining, but it will continue to be hard as long as you are under-medicated.
you should talk to your doctor about symptoms, but leave your weight concerns out of the conversation with your doc. They often see weith gain as a sign of weakness, but, as hypos, we know better. Anyhow, request a dose increase, stating tht you want to keep yur lab values within the normal range, but ath you want to target what a healthy adult wth no thyroid symptoms might have. --A TSH near 1 is a good place to target.
Once you get to that point, if you still have symptoms, or are still gaining weight or unable to lse it, then you may have to get more agressive and/or look into other causes such as Diabetes or Insulin Resistance.
If you could post your lab results that would help. Ideally, a doctor will have TSH, Free T3, and Free T4 tests done. Some docs that are stuck in the past will order other tests instead such as Total T4 and T3 Uptake (NOT an actual T3 lab). The more information you can provide, the more direction we can give you in order to talk to your doctor.
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Danny
I am not a doctor, nor have I ever played one on TV...
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Synthroid causing weight gain? 
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