Effects of early neonatal thyroxine treatment on development of the thyroid and adrenal axes in rats

Neonatal male rats were treated with intraperitoneal (IP) injections of thyroxine on postnatal days 1 to 4. Plasma triiodothyronine (T₃), thyroxine (T₄), and corticosteroids were determined on days 5, 10, 13, 16, 20, 30, and 60 of age, and all animals were weighed weekly. Neonatal thyroxine treatment resulted in reduced body weight throughout the entire test period, decreased T₃ and T₄ until day 60 of age, and elevated corticosteroids on days 5 to 13 of age. These results demonstrate that early neonatal T₄ treatment affects the ontogeny of both thyroid and adrenocortical hormones, and suggest that these developmental changes should be considered when evaluating the consequences of T₄ treatment during the early neonatal period.