The patterns of thyroid development in precocial Japanese quail and altricial Ring doves are described and compared. Thyroid development can be divided into two phases: The first is characterized by increasing functional capacity of the thyroid gland but low circulating concentrations of thyroid hormones; during the second phase there are further increases in thyroid gland activity as well as a shift toward much higher levels of thyroid activity in the periphery. In Japanese quail, the first phase occurs during the latter half of embryonic life, and there is an abrupt transition to the second phase beginning with a perinatal hormone peak. In Ring doves the first phase continues into the first few days of the posthatching period, and the transition to the second phase of higher serum hormones is gradual and lasts until about 6-8 days of age. In addition to the release of hormones from the thyroid gland, serum binding proteins and peripheral tissue 5'-monodeiodinase (which converts thyroxine to triiodothyronine, the metabolically active hormone) play roles in controlling the balance of thyroid hormone availability to the tissues. The potential roles of peripheral deiodinases in hormone dynamics are discussed in relation to tissue and organ growth and maturation during the first phase and in relation to whole organism metabolism with continued growth during the second phase.