Click here to edit the contentWhen it comes to weight gain, nothing beats the thought that thyroid is the first culprit. It is a well-known fact that overactive thyroid can drive weight loss and underactive thyroid can drive weight gain. When you have an underactive thyroid, the metabolic rate of the body slows down. Further due to inadequate thyroid hormone in the body, the person tends to add more water to his body. If low thyroid persists for a long time, weight gain can go on.