Levothyroxine is used to treat an underactive thyroid (hypothyroidism). It replaces or provides more thyroid hormone, which 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 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 (such as thyroid cancer).This medication should not be used to treat infertility unless it is caused by low thyroid hormone levels.