Did you know that levothyroxine and Synthroid can both lead to weight gain? Yes, it's true and the reason this occurs is due to HOW some bodies process T4 thyroid medications. Some people convert levothyroxine and Synthroid into the inactive thyroid hormone which leads to WEIGHT GAIN. Learn how to lose weight if you have thyroid problems and what you can do to stop this from happening in you. You CAN lose weight but it may require using a different thyroid medication and thyroid treatment plan.