Concordant decreases of thyroxine and thyroxine binding protein concentrations during sepsis

The principal thyroxine (T-4) 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 T-4 in sepsis. T-4 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 T-4 binding power in these sera. Total serum binding of T-4, calculated from the normal contribution of each protein to T-4 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 T-4 was proportional to the decrease of serum T-4 from a mean of 8.4 mu g/dL in the normal controls to 4.7 mu g/dL (56% of the control) in the septic patients. Because TBG binds most of the serum T-4, the decrease in the TBG concentration was the major factor in the decrease of total T-4 binding power. Since the concentration of the major T-4 binding proteins decreased sufficiently to account for the decrease in serum T-4, it is unnecessary to postulate the effects of additional factors such as binding inhibitors or modification of T-4 binding protein affinity. Copyright (C) 2000 by W.B. Saunders Company.