|Short description Synthroid (Thyroxine) is used to treat an underactive thyroid (hypothyroidism). It replaces the hormone that is normally produced by the thyroid gland. Low thyroid levels can occur naturally or when the thyroid gland is injured by radiation/medications or removed by surgery. It is important to have adequate levels of thyroid hormone in your bloodstream to maintain normal mental and physical activity. In children, adequate levels of thyroid hormone are important for normal mental and physical growth and development.This medication is used to treat other types of thyroid disorders as well (e.g., certain types of goiters, thyroid nodules, thyroiditis, thyroid cancer).