Effects of transient hypo- and hyper-thyroidism on growth performance, organ weights and serum levels of thyroid hormones in broiler chickens

In order to investigate the effects of transient hypo- and hyperthyroidism on growth performance, organ weights and serum thyroid hormones of broilers, 120 one-day-old broiler chicks were randomly divided into four dietary treatments for six weeks. The dietary treatments included: 1) control, 2) hypothyroid (hypo; propylthiouracil (PTU)-treated), 3) hyperthyroid (HYPER; thyroxine (T sub(4))-treated) and 4) hypo-hyper ((PTU-T sub(4))-treated) groups. PTU and T sub(4) were administered between the ages of 14 to 28 days. Furthermore, a group of PTU treated birds were restored by administering T sub(4) between 28 and 35 days of age to form the hypo-hyper group. In the whole experiment, body weight gain and feed intake were significantly (P < 0.01) decreased by dietary inclusion of PTU and T sub(4) when compared with control birds. Induction of hyperthyroidism significantly impaired feed conversion ratio when compared with other groups (P < 0.01). The relative weight of liver was significantly greater for hypo and hyper when compared with control and hypo-hyper groups (P < 0.01). Induction of hyperthyroidism resulted in decreased abdominal fat when compared with other treatments (P < 0.01). Serum levels of T sub(3) and T sub(4) were significantly influenced by hypo- and hyper-thyroidism in 28, 35 and 42 days of age (P < 0.01). In conclusion, although serum levels of thyroid hormones were affected by dietary treatments, manipulation of thyroid status could not improve growth performance.