The contribution of thyroxine-binding prealbumin to the binding of thyroxine in human serum, as assessed by immunoadsorption.

An immunoadsorption technique employing a rabbit antiserum specific for human serum prealbumin has been devised to remove thyroxine (T(4))-binding prealbumin (TBPA) from serum completely without affecting the T(4)-binding activity of thyroxine-binding globulin (TBG) or the concentration of the other major proteins in serum. As judged from the proportion of T(4) associated with the antigen-antibody precipitate, only about 15% of the endogenous T(4) is bound by TBPA, a value considerably less than that indicated by electrophoretic methods. As judged from the increase in the proportion of free T(4) that followed immunoadsorption of TBPA, TBPA does act as one determinant of the proportion of free T(4) but is far less important than TBG in this respect. A decrease in the T(4)-binding capacity of TBPA cannot solely account for the increase in the proportion of free T(4) in the sera of ill patients, since a comparable increase does not occur in normal sera after complete removal of TBPA. From data obtained in normal and abnormal sera before and after immunoadsorption of TBPA, estimates of the equilibrium constants for the interactions between T(4) and its binding proteins, as they exist in serum, have been derived. The values obtained were: K(ALB), 6.2 x 10(5); K(TBPA), 2.3 x 10(8); and K(TBG), 1.7 x 10(10).