Uses: Levothyroxine is used to treat an underactive thyroid (hypothyroidism). It is a man-made substance that replaces the natural substance (thyroid hormone) that is normally produced by the thyroid gland. Low thyroid hormone levels can occur naturally or when the thyroid gland is injured by radiation/medications or removed by surgery. Having enough thyroid hormone in your bloodstream is important for maintaining normal mental and physical activity. In children, having enough thyroid hormone is important for normal mental and physical development. This medication is also used to treat other types of thyroid disorders (e.g., certain types of goiters, thyroid cancer). It is also used to test thyroid activity.