Concordant decreases of thyroxine and thyroxine binding protein concentrations during sepsis

The principal thyroxine (T4) binding proteins were measured in 8 septic patients and 8 controls to determine the extent to which a decrease in their concentration contributes to the decrease in serum T4 in sepsis. T4 binding globulin (TBG) evaluated by radioimmunoassay (RIA) and radial immunodiffusion (RID) was 61% and 66%, respectively, of the normal mean value in sera from septic patients. Decreases of albumin and transthyretin (TTR) to 55% and 29%, respectively, of the normal mean concentration contributed to the loss of T4 binding power in these sera. Total serum binding of T4, calculated from the normal contribution of each protein to T4 binding and the reduction of its concentration in septic patients, was 55% of that in the normal controls. This decrease in the estimated protein binding of T4 was proportional to the decrease of serum T4 from a mean of 8.4 microg/dL in the normal controls to 4.7 microg/dL (56% of the control) in the septic patients. Because TBG binds most of the serum T4, the decrease in the TBG concentration was the major factor in the decrease of total T4 binding power. Since the concentration of the major T4 binding proteins decreased sufficiently to account for the decrease in serum T4, it is unnecessary to postulate the effects of additional factors such as binding inhibitors or modification of T4 binding protein affinity.