INTERACTION BETWEEN THYROID HORMONE AND THYROXINE-BINDING PROTEINS IN THE EARLY NEONATAL PERIOD

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Abstract
SUMMARY: (1) The interaction between thyroid hormone and thyroxine-binding globulin (TBG) has been studied in 39 pigs from eight litters, aged from 12 hr. to 11 days. In addition, single observations were made on six batches of 42 pigs from 1 day to 6 months old and in five newborn sheep. (2) A rapid increase in the level of the unsaturated TBG capacity was found to occur from the 1st day after birth. (3) Continuous changes in the unsaturated TBG capacity resulted in a decline in the plasma level of free thyroxine and in a decrease in the uptake of [131I]triiodothyronine by erythrocytes. Equilibrium between the hormonal iodine metabolism and its blood carrier is reached at the end of the 1st week of life. (4) These findings suggest that in discussing thyroid metabolism in neonatal animals it is important to distinguish between the early neonatal adaptive period and the remainder of the period of thyroid hyperactivity. The physiological implications of the higher level of free thyroxine in the blood after birth for the adaptation to post-uterine life is discussed.