Thyroid hormones, their activation, degradation and effects on metabolism.

The control of both metabolism and developmental events by thyroid hormones involves a variety of metabolic modifier effects. These actions of thyroid hormones are mediated by nuclear thyroid hormone receptors that have their highest affinity for triiodothyronine (T3). Thyroid hormone deiodinations can have activating or inactivating effects, with the production of T3 (the most active form) and its degradation being of key importance. At the organismic level, the development of thermogenic metabolic responses necessary for homeothermy in birds is correlated with the pattern of thyroid development and the extrathyroidal deiodinations of thyroid hormones. At the cellular level, deiodination effects on cellular T3 availability can protect T3 supply for critical developmental processes (e.g., in brain) or potentially play a role in the differentiation and maturation of various hormone-responsive tissues. Thyroid hormones, both alone and through interactions with other hormones and growth factors, also can influence intermediary metabolic responses that, in turn, play roles in growth and development.