Thyroxine is derived from the amino acid tyrosine:
Both T3 and T 4 are basically two tyrosine molecules with the addition of iodine in either three or four positions respectively.
Tyrosine is one of 20 amino acids and only the L isomer is found in the proteins of mammals. It does not need to be provided by the diet of humans as it can be synthesized in the body from phenylalanine.
Tyrosine also forms the basis for the adrenal hormones and the skin and hair pigment, Melanin.