Thyroid activity of selected, nonselected, and dwarf broiler lines.

Growth rates and serum thyroid hormone concentrations were compared in two trials. In Trial 1 Athens-Canadian (AC) randombred chickens (nonselected) were compared with commercial broiler line chickens (selected). Trial 2 involved the same nonselected and selected lines and a sex-linked dwarf line developed from the same background as the AC population. Selected line birds were heavier than nonselected line birds and nonselected line birds were heavier than dwarf line birds from 1 through 8 weeks of age. Overall, the selected line had lower 3,5,3'-triiodothyronine (T3) concentrations than the nonselected line. No significant differences in thyroxine (T4) concentrations were observed between selected and nonselected lines. The dwarf line had lower T3 and higher T4 concentrations than the other lines. The data suggest a lower rate of T4 deiodination in the dwarf line.