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 T4, T3, rT3, 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 post-hatch animals. In 18-day-old embryos glucocorticoids increased plasma T3 and decreased plasma T4, rT3, 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-1) activity. Glucocorticoid challenge in 8-day-old chicks similarly decreased plasma T4 and the TSH index but tended to also lower plasma T3. 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 T4 secretion in both pre- and posthatch chickens but have clearly different effects on peripheral thyroid hormone deiodination and hence on circulating T3 at both developmental stages.