Triiodothyronine (T3) is one of the two most important thyroid hormones. It is partly formed directly in the thyroid gland, but predominantly it is produced by a process derived from the amino acid tyrosine, for which selenium is required. This production takes place to a large extent in the liver, which is why the production of thyroid hormones can also be impaired in alcoholics by way of liver stress.
T3 is significantly more effective overall than thyroxine (T4), especially at receptors in the cardiovascular system. Its activity is about three to five times higher than that of T4. The most important properties of T4 are: