Plasma thyroid hormone levels and iodothyronine deiodinase activity following an acute glucocorticoid challenge in embryonic compared with posthatch chickens

Embryonic chickens (Day 18 of incubation) and 8-day-old posthatch chicks were subjected to an acute glucocorticoid challenge by a single iv injection of corticosterone (B), dexamethasone, or porcine adrenocorticotropin. Plasma samples were analyzed for changes in T-4, T-3, rT(3), alpha-subunit, LH, GH, and B levels; iodothyronine deiodinase activity was measured in liver, kidney, and hypothalamus at several time points after injection. The effects of the different treatments were broadly similar within one age group, but differed clearly between pre- and posthatch animals. In 18-day-old embryos glucocorticoids increased plasma T-3 and decreased plasma T-4, rT(3), and the calculated TSH index, within hours after injection. These changes were accompanied by an immediate (1-4-24 hr after injection) decrease in hepatic inner ring deiodinating type III enzyme (IRD-III) activity and a delayed (24-48 hr after injection) increase in hepatic outer ring deiodinating type I enzyme (ORD-I) activity. Glucocorticoid challenge in 8-day-old chicks similarly decreased plasma T-4 and the TSH index but tended to also lower plasma T-3 Hepatic ORD-I activity decreased within 1 hr after injection, while the already very low hepatic IRD-III activity was for the most part unaffected. Acute increases in glucocorticoids lower thyroidal T-4 secretion in both pre- and posthatch chickens but have clearly different effects on peripheral thyroid hormone deiodination and hence on circulating T-3 at both developmental stages. (C) 1996 Academic Press, Inc.