Responses to thyrotropin during development in Japanese quail.

Japanese quail, a species with a precocial pattern of development, were used as a model for studying the ontogeny of thyroid responses to TSH. Thiourea was administered to embryos early in incubation, and the effects were assessed by measuring thyroid 32P uptake, serum thyroid hormones, thyroid weights, and body weights. Thyroid stimulation after thiourea treatment indicated that maturation of thyroid-pituitary negative feedback occurs between days 9 and 10 of the 16.5-day incubation period. Since thyroid function continues to increase faster than thyroid size, hypothalamic or higher control of the pituitary appears to dominate over thyroid-pituitary feedback effects for the remainder of the embryonic period. TSH administration to embryos and chicks resulted in similar time-course and dose-response characteristics, as judged by the thyroid 32P uptake response. Single injections of TSH into 14-day-old embryos resulted in parallel increases in serum T3 and T4, with essentially no change in the T3 to T4 ratio. In contrast, the posthatching response of both 1-day-old chicks and adults to TSH involved significant increases in only T4 and resulted in decreased serum T3 to T4 ratios. Thus, there are developmental changes in the hormonal response to TSH. These results indicate that TSH effects on the embryonic thyroid cannot account for the increase in the T3 to T4 ratio during the perinatal hormone peak. Our data are consistent with the idea that T3 produced peripherally by monodeiodination of T4 accounts for the perinatal change in the thyroid hormone ratio.