Evidences gathered over the past ten years indicate that triiodothyronine (T3) is the thyroid hormone responsible for the metabolic effect (1). However, there are particular situations in which little or no T3 is produced by the organism and, therefore, it is not clear how the thyroid action will be expressed. One of these situations is the fetal or neonatal period, in humans and in various experimental animals. Very low amounts of T3 are produced during this time, since the 5’-monodeiodinase is not yet fully developed (2–4). Therefore, thyroxine (T4) is the only thyroid hormone present in large quantities during a period of great hormonal need because of the rapid maturation of several target tissues. Such a situation is particularly evident in chick embryo where most of the target organs, particularly the nervous system, develop between 7–12 days (5), when the concentration of T3 in the serum is very low (6).