The effects of different iodide availabilities on thyroid function during development in Japanese quail

Japanese quail (Coturnix japonica) were used to study the effects of different egg iodide (I) availabilities on thyroid function during development. Low (less than 50 micrograms 1/kg feed in the maternal diet) and high (1200 micrograms 1/kg feed) I availability were compared to control levels (800 micrograms 1/kg feed), a standard supplementation for game bird feed. We measured thyroid gland content of I, triiodothyronine (T3) and thyroxine (T4), plasma concentrations of T3 and T4, hepatic 5' monodeiodinase (5'-D) activity, and the response of the thyroid gland to thyrotrophin (TSH) stimulation. Embryos, on day 14 of the 16.5-17 day incubation period, and 1-day chicks were used for most studies but thyroid gland hormone content and plasma hormone concentrations were determined for more stages. With high I, thyroidal I content was elevated but thyroidal T4 and T3 were not different from controls. Plasma T3 and T4, the thyroid gland response to TSH stimulation, and hepatic 5'-D activity did not differ between control and high I. Reduced body weight occurred with high I. In general, thyroid gland weight was not altered, but some high I birds exhibited thyroid hypertrophy and altered thyroid gland function. With low I availability, thyroid gland contents of I and T4 were reduced but thyroidal T3 content was maintained. The thyroid gland response to TSH stimulation, plasma thyroid hormone concentrations, and the developmental patterns of plasma thyroid hormones, hepatic 5'-D activity, body weight and thyroid weight were not different between control and low I groups. Developing Japanese quail exhibit excellent ability to adjust thyroid function over a wide range of I availabilities. Regulation appears to occur at the level of thyroid hormone synthesis in the thyroid gland, which allows most aspects of thyroid dynamics to remain unchanged in the maintenance of circulating thyroid hormone concentrations.