I Am A 52 Year Old Post Menopausal Woman And

I am a 52 year old post menopausal woman and I have been taking levothyroxine 37.5mcg/day for <7 years. Over the last year, I have gained 20 lbs., for little to no apparent reason. I haven't changed my eating, nor added or taken away any exercise. Do you know what could cause the weight gain and have you any solution to getting rid of it?