Chapter 31 - Thyroid gland

Abstract

Thyroid hormones (THs) are iodinated peptides synthesized and stored by the thyroid gland. Their structure is identical in all vertebrates, and most of their functions have also been conserved throughout vertebrate evolution. THs are transported throughout the avian circulation by TH distributor proteins and are taken up into the cells via transmembrane transporters. Intracellular deiodinase enzymes activate and inactivate THs according to the local needs. The major route of TH action is through binding to nuclear receptors, leading to changes in the transcription of TH-responsive genes, but rapid nongenomic actions also occur. THs are essential for avian development and growth, and they are important regulators of metabolism and reproduction. Changes in the environment such as temperature, light, and food availability influence TH production, metabolism, and action. Multiple contaminants present in the environment unfortunately also influence thyroid function and as such disturb avian development and posthatch life in a persistent way.