Tri-iodothyronine (T 3 ) has been administered in the diet, from day of hatch until 8 weeks of age, to sex-linked dwarf and normal chicks of both sexes from a brown-egg slow-growing strain. Feed was supplemented with either 0.1 ppm or 0.5 ppm T 3 . A significant genotype by treatment interaction was observed on body weight: the effect of T 3 in males was significantly positive for dwarfs and null for normals, the effect in females was null for dwarfs and significantly negative for normals. Feed efficiency was rather decreased by the treatment in both genotypes. Abdominal fatness was decreased in a dose-dependent manner in both genotypes, while rectal temperature was raised in dwarf chicks only. Plasma T 3 was raised to normal levels in dwarfs receiving 0.1 ppm exogenous T 3 , while the 0.5 ppm dose yielded hyperthyroid levels. Plasma GH levels were decreased in a dose-dependent manner by the T 3 treatment, the effect being larger in dwarfs. Surprisingly, plasma IGF-I was unchanged in spite of the GH decrease, whatever the genotype or the sex. It was concluded that exogenous T 3 alone can have a stimulatory effect on growth in dwarfs but can not fully restore a normal growth rate. Both T 3 and IGF-I are important for a normal growth and the relationships between T 3 and IGF-I production should be further investigated in order to better understand the physiological modifications due to the sex-linked dwarf gene.
