Evolution of Thyroid Function and Its Control in Lower Vertebrates

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Abstract
The hypothesis is discussed that the general endocrine function of thyroid hormone in lower vertebrates is concerned with growth, development, and reproduction. Examples are given of possible heightened thyroid function during segments of the life cycles of lamprey, Pacific salmon, some amphibians, and some reptiles. Studies of the occurrence of components of the thyroid system in vertebrate classes suggest that pituitary control of the thyroid through thyroid stimulating hormone may be absent in agnathans and present in gnathostomes. The hypothesis is discussed that regulation of thyroid endocrine function through control of hormone metabolism and target tissue sensitivity is a phylogenetically ancient mechanism.

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