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  • Title: A decrease in the equilibrium constant for the binding of thyroxine to TBG in pregnant women.
    Author: Nakagawa T, Matsumura K, Shinoda N, Toyota S, Tashiro T, Matsushita T.
    Journal: Radiat Med; 1989; 7(6):265-70. PubMed ID: 2517349.
    Abstract:
    The interaction between thyroxine (T4) and thyroxine-binding globulin (TBG) was examined in the serum of pregnant subjects and compared with that in normal serum. Serum T4, TBG capacity, and serum albumin concentrations were measured in serum from six normal and five pregnant subjects. The percent of free T4 (% free T4) was also measured by equilibrium dialysis against veronal buffer in which T4-binding by TBPA was completely abolished. Calculated values for the net equilibrium constant for the binding of T4 to TBG (KTBG) were significantly lower in serum from pregnant subjects (1.18 x 10(10) M-1) than in that from normal subjects (1.65 x 10(10) M-1). To assess the possibility that these findings were due to the presence of a dialyzable inhibitor of T4-binding in the serum from pregnant subjects, serum from normal subjects was dialyzed against that from pregnant subjects. In addition, serum from pregnant subjects was dialyzed against normal serum. KTBG was not altered after dialysis and no evidence of a dialyzable inhibitor of binding was found. The data suggest, therefore, that the TBG in the serum of pregnant women may have a lesser intrinsic affinity for T4 than that in normal serum.
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