Abstract
The human thyroid gland secretes thyroid hormones under the control of the hypothalamus-pituitary-periphery feedback axis already during the last fetal trimester and is the only endogenous source of thyroid hormones after postnatal weaning. The main secretory thyroid hormone product is L-thyroxine (T4) and to a varying extent 3,30,5triiodo-L-thyronine (T3). The latter iodothyronine exerts most of the effects of thyroid hormones on development, differentiation and metabolic pathways via binding to ligand-modulated nuclear or mitochondrial T3-receptors. Direct hormone effects have also been described for T4 and its deiodination products T3 and 3,30,50-triiodothyronine (reverse-T3, rT3) at the plasma membrane and for 3,5-T2 at the mitochondrial level. T4 and its derivatives undergo extensive metabolic reactions in several tissues, organs and cells, the maiority of which are catalyzed by the selenoenzyme family of iodothyronine deiodinases [1]. The three deiodinase isozymes so far identi®ed are expressed in tissue-speci®c and developmentally regulated patterns thus controlling local production and degradation of the thyromimetic hormone T3 and of the other lower iodinated iodothyronines. The deiodinases act as gatekeepers of access of T3 to its intracellular receptors and modulate local availability of T4 and the other iodothyronines for their interaction with cellular targets different from T3-receptors [2]. Type I iodothyronine 50-deiodinase (50DI) preferentially deiodinates T4 and reverse-T3 at the 50-position of the phenolic ring, but also removes iodide from the 5position of the tyrosyl-ring under certain reaction conditions. Type II 50-deiodinase (50DII) deiodinates T4 and with lower af®nity also rT3 speci®cally at the phenolic ring in 50-position. The type III iododthyronine 5-deiodinase (5DIII) removes iodide from the 5-position of the tyrosyl ring and thus catalyzes the inactivation of the thyromimetic activity of T4 and T3.
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