or l-tetraiodothyronine or T 4
One of the two major hormone s (along with the closely related l-triiodothyronine, or T 3 ) secreted by the thyroid gland .
Its principal function is to stimulate oxygen consumption and thus metabolism in all cells and tissues in the body. Thyroxine is formed by the addition of iodine to the amino acid tyrosine while the latter is in a protein-bound form. Thyroxine secretion is excessive in hyperthyroidism and deficient in hypothyroidism.