Propylthiouracil (PTU) is a thioamide drug used to treat hyperthyroidism. It is a medicine that is used to decrease the amount of thyroid hormone produced by the thyroid gland. PTU inhibits many steps in the synthesis of thyroid hormones, including the addition of iodide to thyroglobulin by the enzyme thyroperoxidase, a necessary step in the synthesis of thyroxine, and by inhibiting the enzyme 5'-deiodinase which converts T4 to T3.