[Evaluation on the interaction between thyroxine binding globulin (TBG) and thyroxine (T4) in healthy subjects and pregnant women].

A very simple method for the evaluation of the interaction between thyroxine binding globulin (TBG) and thyroxine (T4) has been developed and evaluated in 6 healthy subjects and 5 pregnant women. Rabbit anti-human TBG antiserum was precipitated with 50% ammonium sulfate followed by passing a DEAE-cellulose column. Immunoglobulin G (IgG) fraction thus prepared was covalently attached on a Sepharose CL-4B and used as an immunoadsorbent of serum TBG. After treatment with charcoal to remove endogeneous thyroid hormones, 10 microliter of sera from 6 healthy subjects and 5 pregnant women were incubated with 125I-T4 and various concentrations of cold T4 followed by immunoadsorption with anti-TBG-Sepharose, and association constants (Ka) between TBG and T4 were calculated in each subject using Scatchard's plot. The Ka value thus obtained in 6 healthy subjects was 1.47 X 10(8) M-1 (SD = 0.24), and in 5 pregnant women it was 0.82 X 10(8) M-1 (SD = 0.21) which was significantly lower than that of healthy subjects (P less than 0.001). Our present method does not need purification of TBG from serum to investigate its functions, especially in relation to the binding with thyroid hormones. Also, only a very small amount (100 approximately 150 microliter) of serum is enough for the calculation of Ka between T4 and TBG. The whole procedure is very easy to perform and can be done in a relatively short time, and therefore we consider this method clinically relevant and useful.