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Influence of feeding level during postweaning growth on circulating concentrations of thyroid hormones and extrathyroidal 5'-deiodination in steers.

An experiment was conducted with 42 growing Montbéliard steers to study the effect of feed restriction, followed by refeeding, on circulating concentrations of thyroxine (T4) and triiodothyronine (T3) and on hepatic and muscle activities of 5'-deiodinase (5'D). At 9 mo of age, 21 steers were diet-restricted for 3 mo (ADG, 641 g/d), prior to a 4-mo compensatory growth period with ad libitum access to the same diet (ADG, 1,240 g/d). They were compared to 21 control steers continuously gaining 1,100 g/d between 9 and 16 mo of age. Blood samples were collected every 14 d and samples of liver and semitendinosus and triceps brachii (triceps) muscles were obtained at slaughter at the end of the restriction and refeeding periods (12 and 16 mo of age, respectively). Compared to control steers, feed restriction decreased plasma concentrations of T4 after 56 to 83 d of feed restriction (P < 0.05), whereas T3 concentration decreased only after 83 d of feed restriction (P < 0.05). No differences in hepatic and muscle 5'D activities were observed after 87 d of feed restriction and decreased growth rate (12 mo of age). During the refeeding period (compensatory growth), circulating concentrations of T4 and T3 were restored to control levels within 14 d. Moreover, T3 concentration rose above that of control steers after 56 d of refeeding and remained higher for the duration of the experiment (P < 0.05). Hepatic 5'D activity was higher (P = 0.07) in compensated than in control steers at the end of refeeding period (16 mo of age) and higher (P < 0.01) after compensation at 16 mo than during restriction at 12 mo. Activities of 5'D in semitendinosus and triceps muscles were higher (P < 0.001) in 16-mo-old than in 12-mo-old steers, but no differences were observed due to feed restriction or compensatory growth. These results indicate that nutritional status regulates both thyroidal secretion and extrathyroidal T3 production in cattle. The data also suggest that extrathyroidal T3 production may be involved in the mechanism of compensatory growth in cattle.