HI, thanks for using healthcare magic There are no known interactions between cipralex and levothyroxine (if this is the thyroid hormone you are using). In terms of weight gain , there are some medications that can cause weight gain. They can do this in different possible ways including: by stimulating the appetite or decreasing energy levels or decreasing fullness or affecting metabolism. You can counteract this by using healthy foods and increasing your activity level. You can only gain weight if when stimulated to eat, you eat foods high in calories or certain carbohydrates. I hope this helps