Effect of dietary thyroxine on growth and muscle protein metabolism in broiler chickens

Effects of induced hypo- and hyperthyroidism on immune function and plasma biochemistry in mallards (Anas platyrhynchos)

Hypo- or hyperthyroid states were induced in adult male mallards (Anas platyrhynchos) by subchronic exposure to daily injections of methimazole or a 9:1 ratio of thyroxine (T4): triiodothyronine (T3). The levels of T4 given were 0, 125, 250, or 500 micrograms/kg/day and for methimazole; 10 mg/kg/day for 22 or 21 days. Plasma T3 showed a lasting decrease with T4:T3 treatment, despite the attempt to maintain the normal T4:T3 ratio. Antibody formation to sheep red blood cells was decreased only at the 125 micrograms/kg/day dose of T4, and was unaffected by methimazole treatment. Natural killer cell activity to RP-9 tumor cells and macrophage phagocytosis of killed, opsonized Saccaromyces cereviseae were unaffected by treatment throughout the study. However, lectin-dependent cellular cytotoxic activity to RP-9 tumor cells was significantly decreased after 21 days of methimazole treatment, indicating that hypothyroidism may have an influence on cell-mediated immunity. Hypo- and hyperthyroid conditions had opposing effects on plasma cholesterol levels.

Interaction of thyroxine and testosterone in stimulating muscle protein breakdown in female broiler chickens

1. The effects of thyroxine (T4), testosterone propionate (TP) and a combination of hormone treatments on growth and the rates of muscle protein breakdown estimated by Ntau-methylhistidine excretion were studied. 2. Broiler chickens aged from 15 d to 27 d were used; T4 and TP were mixed in the basal diet at concentrations of 1.2 and 20 mg/kg respectively. 3. Although growth was not altered significantly by any treatment, the rate of muscle protein breakdown was significantly increased about 2-fold by the combined treatment, but not by treatment with T4 or TP alone.