Thyroxine, also tetraiodothyronine, main hormone secreted by the thyroid gland. The function of this and the other thyroid hormone, triiodothyronine, is to increase the cellular rate of carbohydrate metabolism and of protein synthesis and breakdown. The hormones are synthesized in the thyroid by combining iodine with the amino acid tyrosine and are secreted into the blood as a complex with the protein plasma globulin. Both synthesis and secretion are regulated by, and in turn regulate the formation of, a hormone secreted by the pituitary gland. Thyroxine was first isolated in 1919 by the American biochemist Edward Calvin Kendall and was synthesized in 1927 by the British biochemist Charles Harington. Synthetic thyroxine is now used in treating thyroid-deficiency conditions such as cretinism and goiter.