As the serum concentrations of T4 remained unaltered, and the avian thyroid secretes almost exclusively or at least more than 99% of T4, the bioactive T3 has to be produced in the extrathyroidal tissues, essentially by deiodination (Darras et al., 2006).
In the chicken, the sharp increase in plasma T3 prior to hatching is reported as a combined result of an increase in hepatic D1 activity and a decrease in hepatic D3 activity (Borges et al., 1980; Galton and Hiebert, 1987; Darras et al., 1992b).