Effects of dietary thyroid hormones on growth and serum T3, T4, and growth hormone in sex-linked dwarf chickens.

Dwarf pullets fed with either T3 or T4 at 0.1, 1.0, and 10.0 ppm for 2 weeks showed no improvement in their body weight gain as compared with birds that were fed a control diet. Birds fed T3 or T4 at 10.0 ppm showed poorer growth, body weight gain, and feed efficiency than control birds. Pullets fed 1.0 ppm of T3 showed significantly better feed efficiency than control birds. Serum T3 concentrations were significantly higher when birds were fed 1.0 ppm of T3 or 10.0 ppm of T3 or T4. Plasma T4 concentrations were also higher in T4 fed birds (1.0 ppm and 10.0 ppm), but were significantly lower in T3 fed birds (1.0 ppm and 10.0 ppm) than in control birds. In birds fed T3 or T4 at 1.0-ppm and 10.0-ppm levels serum growth hormone concentrations were significantly lower as compared with control birds. In conclusion, exogenous T3 and T4 did not correct the sex-linked dwarfism of dwarf chickens. Such dwarfism is characterized by low circulating levels of T3 and T4.