Many people experience weight gain at some point in their lives. In most cases, it is linked to lifestyle factors such as diet, lack of exercise, or stress. However, unexplained weight gain—when your weight increases without significant changes in diet or activity—may indicate an underlying health issue. One of the most common medical causes of unexplained weight gain is a problem with the thyroid gland. The thyroid controls how your body uses energy, and when it does not function properly,...