Thyroid Hormone Physiology in Galliformes: Age and Strain Related Changes in Physiological Control

SYNOPSIS. Considering thyroid hormone levels in the chicken, we should keep in mind that these concentrations are the integrated result of production and release on one hand and elimination, tissue utilization or peripheral interconversion on the other. Moreover all these processes can be and are probably influenced by the relative amounts of thyroxine (T4), triiodothyronine (T4) and reverse-triiodothyronine (rT4) as well as by the ratio of free/bound thyroid hormones. Additionally, other hormones will act upon the thyroid axis and its peripheral metabolism and vice versa. Environmental factors directly or indirectly modify or modulate this already complicated picture. In this review, we therefore will first mention the circulating thyroid hormone levels and the thyroid content of iodohormones. In the second section, the control by the central axis of thyroxine production by the thyroid gland as well as some aspects of peripheral metabolism, with the attention focussed on thyroxine monodeiodination is discussed. Processes which are known to be of importance in thyroid physiology such as recompartementalization or bile excretion are hardly studied in avian species. Many questions about the control of thyroid functioning remain unanswered because of the lack of a homologous TSH radioimmunoassay. It is tempting to assume that the mammalian model can give us the answers; however, too many exceptions in regard to thyroid hormone physiology as it is known in mammals are described, thereby underscoring that thyroid physiology in avians should be investigated without a bias towards research done in mammals.