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 integratedresult 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 influencedby the relative amounts of thyroxine (T₄), triiodothyronine(T₄) and reverse-triiodothyronine (rT₄) as well as by the ratioof free/bound thyroid hormones. Additionally, other hormoneswill act upon the thyroid axis and its peripheral metabolismand vice versa. Environmental factors directly or indirectlymodify or modulate this already complicated picture. In this review, we therefore will first mention the circulatingthyroid hormone levels and the thyroid content of iodohormones.In the second section, the control by the central axis of thyroxineproduction by the thyroid gland as well as some aspects of peripheralmetabolism, with the attention focussed on thyroxine monodeiodinationis discussed. Processes which are known to be of importancein thyroid physiology such as recompartementalization or bileexcretion are hardly studied in avian species. Many questionsabout the control of thyroid functioning remain unanswered becauseof the lack of a homologous TSH radioimmunoassay. It is temptingto assume that the mammalian model can give us the answers;however, too many exceptions in regard to thyroid hormone physiologyas it is known in mammals are described, thereby underscoringthat thyroid physiology in avians should be investigated withouta bias towards research done in mammals.