L-Thyroxine (sodium salt pentahydrate) (Standard) is the analytical standard of L-Thyroxine (sodium salt pentahydrate). This product is intended for research and analytical applications. L-Thyroxine sodium salt pentahydrate (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].