Enzymatic deiodination of thyroid hormones

Thyroid honnone plays an essential role in the regulation of cell growth, differentiation and metabolism in vertebrates. These actions are initiated by binding to the nuclear thyroid honnone receptor that has been identified as the translation product of the proto-oncogene c-erbA, the cellular counterpart of the viral oncogene v-erbA. Binding of thyroid honnone to its receptor changes the interaction with cis-acting regulatory elements, that modulate the expression of responsive target genes. There are several thyroid hormone receptor isofonns that exert regulation in an unexpected degree of complexity. Although the mechanisms of actions of thyroid honnone in various tissues are considered to be largely nucleus-mediated, several actions of this honnone are very likely extranuclear.