L-Thyroxine-13C6-1 (Levothyroxine-13C6-1, T4-13C6-1) is a 13C labeled L-Thyroxine (HY-18341). L-Thyroxine (Levothyroxine, T4) is a synthetic hormone for the research of hypothyroidism. DIO enzymes convert biologically active thyroid hormone (Triiodothyronine,T3) from L-Thyroxine (T4)[1][2].