As I've thyroid, I'm taking l-thyroxine 100mcg medicine since one month. Pls tell me if there are any side effects of this medicine like weight gain..

No side effects. Thyroxine is a replacement hormone that takes the place of your naturally occurring hormone. Thyroxine can give expected effects if too much or too little is taken but these are not "side effects." keep your TSH on the low end of normal - say below 2.5 and all will be well.