Thyroid function in the newborn lamb. Physiological approach of the mechanisms inducing the changes in plasma thyroxine, free thyroxine and triiodothyronine concentrations.

Several experiments were performed to study the mechanisms inducing the neonatal rises in plasma iodothyronine concentrations in lambs. TSH levels rose during the first 4 to 8h of life, whereas plasma T4 an T3 concentrations increased only from birth to respectively 2 and 1h; the rise in free T4 levels was longer and more important than the rise in total T4. Only T4 changes were strongly related to the extent of TSH increase. The neonatal TSH surge was inhibited by delaying the first milk intake, indicating a great importance of the early nutritional status; in these conditions, the neonatal T4 rise did not occur, whereas the T3 increase was not affected; therefore, in contrast to T4, the T3 increase occurring at birth is not TSH-dependent. As in thyroidectomized lambs continuously infused with T4, plasma T3 concentrations did not increase at birth, it appears that the neonatal T3 surge probably has a thyroidal origin. These results raise the possibility of the existence of a specific stimulator of thyroidal T3 secretion, at least in the newborn lamb. In addition, comparison of the respective T4 increases, at birth or after TSH stimulation in 24 h-old animals, suggests that the ability of the thyroid to respond to a sustained stimulation is strongly reduced at birth. Lastly, neonatal changes in the affinity and/or capacity of carrier proteins for T4, perhaps partly induced by the observed simultaneous rise in free fatty acid levels, could explain that plasma T3 concentrations remained elevated despite a decrease in total T4 levels from 2 h after birth.