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Jack H. Oppenheimer
University of Minnesota, Minnesota, MN, USA Thyroid hormone has diverse functions which are best understood in the context of evolutionary principles (1). The effect of thyroid hormone on metabolism and thermogenesis is a relatively late development, associated with the differentiation of birds and mammals. The function of thyroid hormone in earlier forms appears less clear but conceivably could have been related to somatic differentiation as dramatically illustrated by the phenomenon of amphibian metamorphosis (2). This function of thyroid hormone is clearly preserved in mammals as is evident in the catastrophic effects of fetal thyroid hormone deficiency in human beings, an effect which in its extreme form results in the well-known syndrome of cretinism. On a world-wide basis thyroidal deficiency is most often related to deficient iodine intake. However, whereas borderline intake of iodine may not result in frank cretinism, some observers have suggested that it may still impair intellectual development and prevent the subject from reaching his or her full genetic potential. In the past five years my colleagues and I have turned our attention to an examination of the molecular basis of thyroid hormone action. Our hope in these studies was to develop a better understanding of the developmental actions of the thyroid hormone and, possibly, to generate insights into the overall mechanisms by which other hormones and small molecules could influence the maturational process. Our work in this area has recently been summarized in a review (3). |
