Thyroxine hydrochloride- 13 C 6 is the 13 C-labeled L-Thyroxine. 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).