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Thyroid Hormones, Their Activation, Degradation and Effects on Metabolism¹

Department of Biology, Virginia Tech, Blacksburg, VA 24061-0406

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.

KEY WORDS: • thyroid hormones • metabolism • deiodination • development

¹ Presented as part of the 59th Annual Poultry Nutrition Conference: Metabolic Modifiers, given at the Experimental Biology '94 meeting, Anaheim, CA, on April 24, 1994. This conference was sponsored by the American Institute of Nutrition and was supported by grants from Merck Sharp and Dohme Research Laboratories, Smith Kline Beecham Animal Health Products, Perdue Farms Inc., Kemin Industries Inc., and Prince Agri Products, Inc. The guest editor for this conference was Theodore H. Elsasser, Ruminant Nutrition Lab, U.S. Department of Agriculture, Beltsville, MD.

² To whom correspondence should be addressed: Department of Biology, Virginia Tech, Blacksburg, VA 24061-0406.