Increased sensitivity to triiodothyronine (T3) of broiler lines with a high susceptibility for ascites

1. In the studies reported here, broiler lines divergently selected for susceptibility to ascites under low temperature conditions were tested for their sensitivity to 3,3’,5‐triiodothyronine (T3) with respect to growth rate, rate of mortality, plasma concentrations of T3, right ventricular hypertrophy (RVH) and incidence of ascites.

2. Mean body weight of the ascites‐susceptible line (BC‐line) was higher than that of the ascites‐resistant line (A‐line). Adding 0.5 mg T₃/kg of the diet depressed growth rate to the same extent in both lines. The effect of T3 on growth was more pronounced for males than for females.

3. T3‐supplementation increased the relative weight of the heart and the incidence of RVH to the same extent in both lines. More of the T3‐treated BC‐line chickens had fluid accumulation in the abdominal cavity than the T3‐treated A‐line chickens.

4. Dietary T3‐treatment depressed the plasma concentration of growth hormone (GH) profoundly and insulin‐like growth factor (IGF‐I) slightly but to the same extent in both lines. The coefficient of variation of GH concentrations indicate that T3 treatment mainly decreased GH‐pulsatility in young growing broilers.

5. Higher doses of dietary T3 (1 and 2 mg/kg) increased mortality in a dose‐dependent manner. With 2 mg T₃/kg, mortality in the BC‐line was almost double that in the A‐line.

6. These studies indicate that the development of ascites could be linked with thyroid function. Moreover, dietary T3 supplementation could be used to help identify ascites‐inducing factors or genetic lines with differential sensitivity for ascites.