Postnatal changes in total and free thyroxine and triiodothyronine in foal serum.

Total tyhroxine (TT4), free thyroxine (FT4), total triiodothyronine (TT3) and free triiodothyronine (FT3) concentrations in foal umbilical cord blood were respectively 14, 5 7 and 3 times the concentrations of these hormones in adult horse blood. The TT4 levels in foals declined rapidly to reach adult concentrations by Day 16 and FT4 levels declined steadily during the first 3 months of life. Foal TT3 levels rose during the first 10 hr after birth and thereafter declined, although they were still X2-5 higher than adult levels at 3 months of age. Levels of FT3 similarly increased after birth becoming X8 higher than adult levels at 3 days of age. They declined after this time but were still twice the normal adult levels at 3 months of age. Associated with these age changes were an increase in the free hormone percentage and a decrease in binding protein concentration. The very high levels of thyroid hormones in newborn foals may be responsible for their extremely high thermogenic capacity and rapid pre- and post-natal rates of growth, especially of the musculo-skeletal and nervous systems.