Selenodeiodinases and their role in thyroid hormone activation and inactivation

Deiodination in the thyroid gland and extrathyroidal tissues is the predominant mechanism whereby thyroxine is converted into more active and less active thyroid analogues. The deiodinases that catalyze these reactions are now known to constitute a family of structurally related selenoproteins. This review focuses on progress in our understanding of the physiology, biochemistry, and regulation of these enzymes. Recent developments have provided important information about the structure and expression patterns of the type 2 deiodinase, the mechanisms whereby selenocysteine is incorporated into eukaryotic proteins, the effects of selenium deficiency on thyroid hormone metabolism, the important role of the placenta in controlling fetal thyroid hormone levels, and the role of thyroid hormones, growth factors, and cytokines in regulating deiodinase expression in different tissues. With the availability of immunologic and molecular reagents for studying these enzymes, it is expected that our understanding of their structure, function, and physiologic roles will continue to progress at a rapid pace.