Peripheral metabolism of thyroxine

Abstract The new edition of Werner's The Thyroid is so massive a monograph that anyone might be forgiven for supposing that all must now be light as regards the thyroid and its hormones. Such a conclusion would be much in error: many important areas remain most murky. Some of the problems involved in understanding the metabolism of thyroid hormones at the cellular level received a recent airing in the lecture of Professor Reed Larsen at the 1987 meeting of the British Endocrine Societies (Larsen, 1987). Although it is now generally accepted that the important thyroid hormone for the cells is tri-iodothyronine (T3), it is not clear precisely how the supply is matched to the cellular need, for T3 forms only a small proportion of the thyroid secretion, the predominant hormone secreted being thyroxine (T4). The cellular supply of T3 depends largely on conversion of T4 to T3 by an enzyme