Plasma Concentrations of Thyroid Hormone and Growth Hormone in Lohmann Male Broilers Fed on Different Dietary Energy and Protein Levels

An experiment on Lohmann male broilers was conducted to evaluate the effect of different energy and protein levels on broiler performance and plasma concentrations of growth hormone (GH), thyroxine (T4), and triiodothyronine (T3). 600 day-old broilers were fed on either 2800(low-energy) or 3200(high-energy) Kcal ME/Kg diets with different protein levels during three periods of starter, grower and finisher. Blood samples were collected at the end of each period from wing vein. Plasma levels were assayed for GH, T4 and T3 by radioimmunoassay (RIA). Body weight (BW) and feed conversion rate (FCR) were also measured at the end of each period. FCR and mean concentrations of plasma GH and T3 increased in broilers fed on low-energy diets compared with those fed on high-energy diets during different periods, but BW and mean concentrations of plasma T4 decreased in the three periods in birds fed on low-energy diets compared with those fed on high-energy diets. Birds fed on different protein levels showed significant differences with respect to BW, FCR and mean concentrations of plasma for GH, T3 and T4 during starter, grower and finisher periods. At the two energy levels and all periods, FCR and mean concentrations of plasma GH and T3 were decreased in broilers fed on high-protein diets compared with those fed on low-protein diets. However, higher BW and plasma T4 concentrations were observed in broilers fed on high-protein diets compared with those fed on low-protein diets. The results of this experiment demonstrate that low-energy intake could increase FCR and mean plasma concentrations of GH and T3 but could decrease BW and mean plasma concentrations of T4. Different protein levels during starter, grower and finisher periods in broilers may change the plasma concentrations of GH, T4 and T3 but nearly similar protein levels were not changed (P>0.05) those hormones.