THYROIDAL secretion is the only source of thyroxine (T4) in the human systemic circulation, and in euthyroid man, T4 is the main secretory product of the gland. The metabolic fate of the hormone once thyroidal secretion has occurred is complex and involves deiodination, deamination and decarboxylation of the amino acid side chain, and esterification of the phenolic hydroxyl group with glucuronic or sulfuric acid. These pathways are not to be considered as mutually exclusive, but are interrelated in an exceedingly complex manner. Deiodination of T4 is the most important degradative pathway and accounts for up to 85% of the disposal of the hormone (1–3). At least two major mechanisms exist within the peripheral tissues by which the iodine atoms may be removed from the molecule. Of these, monodeiodination is quantitatively the most important pathway and has been subjected to the most intensive investigation.