Supplemental triiodothyronine, feeding regimens, and metabolic responses by the broiler chicken.

There are conflicting results concerning the role of the thyroid hormones in lipid metabolism. The experiments in this report were designed to examine the role of T(3) in modifying responses obtained by shifting birds from moderate to low protein diets. Birds were grown from 7 to 28 d on a diet containing 18% protein. At this time, birds were switched to a diet containing 12% protein +/- T(3) The switch was accomplished either immediately or after a 24 hr fast. Measurements taken included in vitro lipogenesis (IVL), hepatic enzyme activities and plasma metabolites and thyroid hormones. Simply switching to birds to the low protein diet increased IVL, but rates were similar for three days following the switch. Feeding T(3) in this same regimen resulted in lower, but again, constant rates of IVL. In contrast, although switching protein levels after a 24 hr fast increased IVL, the rate after two days of refeeding was nearly double that following one day. This accentuated response was somewhat attenuated by including T(3) in the diet. Neither fasting nor refeeding altered plasma T(3) relative to ad libitum values. Supplemental dietary T(3) increased plasma T(3) and results were not affected by feeding regimens. Plasma T(4) was greatest in birds fasted for 24 hr and least in birds fed T(3) suggesting that feeding regimens may regulate the conversion of T(4) to T(3) It is suggested from this study that some of the effects of alterations in dietary feeding regimens can be modulated by T(3)