Dietary protein effects on the broiler's adaptation to triiodothyronine

The purpose of this experiment was to study the effects of dietary protein on metabolic adaptations in birds given triiodothyronine (T₃) at 28 d of age. Knowledge about the role of dietary protein in thyroid metabolism is lacking. Male broiler chickens were fed diets containing either 120 or 300 g crude protein/kg from 7 to 28 d. At this time, one-half of each group was given that diet supplemented with 1 mg/kg T₃. Birds were sampled at 0, 2, 5, 7, 9 and 12 d following the initiation of the T₃ treatments. Measurements taken at these intervals included in vitro lipogenesis (IVL), growth and feed consumption, hepatic enzyme activities and plasma thyroid hormones and metabolites. As expected, IVL was greater at 28 d in birds fed the lower level of protein. Moreover, when T₃ was added to either diet, IVL decreased by 50% after 5 d. Plasma IGF-I and T₄ were greater while T₃ and GH were less when birds were fed the higher level of crude protein. Plasma T₃ increased and T₄ decreased 3 d following the addition of T₃ to diets containing either level of crude protein. Most metabolic changes in response to feeding T₃ occurred within 2 to 5 d, suggesting that changes in intermediary metabolism preceded morphological changes that we have previously seen. In addition, dietary protein levels may not affect adaptive responses to T₃.