A role for thyroid hormones in the regulation of diet-induced thermogenesis in birds.

The possible involvement of thyroid hormones in avian diet-induced thermogenesis (DIT) was investigated in two lines of cockerels divergently selected for high (R-) or low (R+) food efficiency. For a given body weight, R+ cockerels exhibited a higher food intake than R- cockerels (+49 to +76%) and increased DIT (+25%). Plasma thyroxine (T4) level did not differ between lines whatever the feeding status of the birds. Plasma 3,5,3'-triiodothyronine (T3) level was lower in fasted R+ than in fasted R- cockerels while the opposite was observed after a meal. Iopanic acid injections reduced both plasma T3 concentrations and heat production to the same levels in both lines. Hepatic 5'-deiodinase activity measured with an exogenous sulfhydryl group (dithiothreitol) did not differ between lines, but when the sulfhydryl group was omitted, the activity was higher in R+ than in R- birds (90 v. 42 pmol T3/min per liver). T3-binding capacity of isolated hepatic nuclei was higher (+76%) in R+ than in R- birds. Long-term or acute pair-feeding of R+ cockerels to the level of R- controls did not alter these results. The present results suggest that T3, mainly originating from peripheral conversion of T4 to T3, is involved in DIT in the R+ line. Availability of endogenous sulfhydryl groups appears to play an important part in the modulation of hepatic deiodinase activity. The higher concentration of nuclear T3 receptors may further increase the effects of the hormone, suggesting a major role of thyroid hormones associated with catecholamines in the stimulation of avian DIT. The underlying thermogenic mechanisms remain to be elucidated.