PHYSIOLOGICAL SIGNIFICANCE OF THYROXINE-BINDING BY PROTEINS: THE RELATIONSHIP BETWEEN CHANGES IN THYROXINE-BINDING GLOBULIN CAPACITY AND THE DAILY UTILIZATION RATE OF THYROXINE IN THE NEWBORN PIG

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Abstract
SUMMARY: (1) Experiments were performed on three litters of pigs (four animals in each litter) between birth and the 8th day (inclusive) of life. (2) The relation between the variation in thyroxine-binding globulin (TBG) capacity and the rate of thyroxine utilization has been taken as an indication of the ability of thyroxine production to maintain a fairly constant percentage of saturation of the TBG. (3) Dynamic changes in the interaction between the thyroid hormone and the plasma protein binding sites are shown. Changes in the binding capacity of TBG were negatively correlated with uptake of [131I]triiodothyronine by erythrocytes (r = −0·64; P < 0·01) and positively correlated with the protein-bound iodine (PBI) value (r = +0·92; P < 0·01). (4) The value for saturation of the plasma TBG capacity was relatively stable and amounted to 67·38 ± 5·47%. (5) The total quantity of thyroxine which has to match the variability of the protein binding site capacity was calculated to be only 13·72% of the daily production of the hormone in 8-day-old pigs. (6) The physiological implication is discussed of changes in TBG capacity resulting in an increase of the bound-thyroxine pool in relation to the daily utilization rate of the hormone.