Day 14 embryos (16.5-17 day incubation period) and 1 day old chicks of Japanese quail (Coturnix japonica) were used to study the effects of different egg iodine (I) availabilities on thyroid function during development. Low (≤50 μgI/kg feed in the maternal diet) and high (1200 μgl/kg feed) I availability were compared to control levels (800 μgI/kg feed). Thyroid gland (TG) content of I, triiodothyronine (T3), and thyroxine (T4), plasma concentrations of T3 and T4, and hepatic 5’ monodeiodinase (5’-D) activity was measured, and the response of the TG to thyrotropin (TSH) stimulation [TG-cAMP content and plasma thyroid hormone (TH) concentrations]. Also, the developmental patterns of TH concentrations in the TG and plasma were determined. With increased I availability, TG-I content is elevated but thyroidal T4 and T3 and the developmental pattern of TG-TH were not different from controls. Plasma T3 and T4 and the developmental pattern of plasma TH were not altered. Indicators of the TG response to TSH stimulation were not different with increased I availability. Hepatic 5’-D activity did not differ between control and high I availability. Reduced body weight was associated with increased I availability. In general, TG weight was not altered, but a small percentage of the high I birds exhibited TG hypertrophy and altered TG function. With low I availability, TG-I content was reduced. Although thyroidal T4 content was reduced on embryonic day 14 and thereafter, TG-T3 was maintained throughout development. The magnitude of the TG response to TSH stimulation was not altered with reduced I availability. Hepatic 5’-D activity, plasma TH concentrations and the developmental patterns of plasma TH were not different between control and low I availability. Reduced I availability did not affect body or TG weight. Developing Japanese quail exhibit excellent ability to adjust thyroid function over a wide range of I availabilities. Regulation appears to occur at TH synthesis which allows most aspects of thyroid dynamics to remain unchanged in the maintenance of circulating TH concentrations. / Master of Science