Low thyroid functioning is often blamed for weight gain. Some investigations suggest that the excess weight may be slowing down the thyroid, instead of a slow thyroid making it easy to pack on the pounds. Understanding the link between weight and thyroid is crucial, researchers say, because obesity raises your risk of getting thyroid cancer. Treating an overweight person with thyroid medication may be the wrong course of action, she says, if in fact the weight gain is what made the thyroid sluggish. The better approach, in some cases, may be weight loss.