Understanding of biosynthesis and metabolism of thyroid hormones have recently been substantially improved via the molecular characterization of key players like proteins essential for hormone biosynthesis, cellular thyroid hormone transporters, and intracellular enzymes involved in their activation in peripheral tissues. This improved our understanding of a number of difficult to interpret laboratory conditions. It further stimulated the development of new techniques for the future sensitive and specific measurement of a much wider than the conventional range of secretory products of the thyroid and their organ specific activation.