Synthetic thyroid hormone molecule. Computer model of a molecule of levothyroxine (L-thyroxine, also called synthetic T4), the synthetic form of the thyroid hormone thyroxine. It is used as a drug and prescribed to treat the thyroid gland disorder hypothyroidism (a deficiency of thyroid hormones). Thyroxine contains the chemical element iodine (I) and plays an important role in regulating body metabolism. The chemical formula of the hormone is C15.H11.I4.N.O4. This molecular model has colour- coded rods for atoms: carbon (light blue), hydrogen (grey), iodine (purple), nitrogen (dark blue), oxygen (red). Levothyroxine is a chiral isomer of natural D-thyroxine, which means that it is metabolised more slowly.