**1. Introduction**

36 Thyroid Hormone

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Hormones are important mediators in the responses of a suite of phenotypic traits to environmental changes. Therefore, populations inhabiting different environments are expected to vary in several hormonal pathways. Such variation results from both plastic response to environments and genetic differences. Therefore, information about the genetic basis of hormonal variation is crucial to better understand the ecological and evolutionary mechanisms of phenotypic diversification in animals. Furthermore, information about the racial and geographical variation in hormone physiology is crucial for better diagnosis of hormone-related diseases in clinical fields. Thyroid hormones play key roles in regulation of many physiological and behavioral traits, such as metabolism, ion homeostasis, basal activity, and longevity. Therefore, thyroid hormone can play important roles in adaptation to external environments. In the present study, we review interspecies, racial, geographical, and interindividual variation in the thyroid hormone pathways in humans and other animals. The present review focuses on natural and subclinical variation in thyroid hormone physiology and will not cover the genetic basis for congenital hypothyroidism [1,2,3,4,5], congenital hyperthyroidism [6,7], autoimmune diseases [8], and thyroid cancers [9], for which a number of good review articles are already available. We also review what is known about the genetic basis for such variation. We found several shared features in the patterns of variation in thyroid hormone physiology in humans and other animals. This review demonstrates the importance of undertaking further integrative studies of human genetics and animal ecology for a better understanding of the ecological and genetic mechanisms of variation in thyroid hormone signaling pathways.

© 2012 Ishikawa and Kitano, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
