Peripheral Thyroid Hormone Dynamics in Precocial and Altricial Avian Development1

Abstract

SYNOPSIS. Precocial and altricial birds have distinctly different patterns of general ontogeny and metabolic/thermoregulatory development. In my laboratory, we have studied the development of thyroid function in Japanese quail ( Coturnix japonica ) and Ring doves ( Streptopelia risoria ) as examples of precocial and altricial development, respectively. In this paper, I review the literature and our work on the factors that influence peripheral hormone dynamics in birds. The first section of the paper describes adult peripheral thyroid function to set the stage for the developmental picture. Thyroid development is divided into two phases: Phase I, in which the thyroid gland develops functional capacity but there is low thyroid activity in the periphery, and Phase II, in which peripheral thyroid function increases and approaches adult levels. In precocial development Phase I occurs during embryonic life and Phase II begins at the perinatal period. In altricial development, the pattern of thyroid functional development is different and delayed. Peripheral thyroid hormone patterns in both developmental modes are discussed with reference to the factors that determine their dynamics: hormone availability from the thyroid gland, total serum hormone concentrations, the roles of serum binding proteins in regulating freehormone concentrations, hormone turnover and excretion, hormone receptors and thyroid hormone metabolism, especially extrathyroidal production of triiodothyronine from thyroxine.