Discrepancy between the measurement of thyroxine-binding prealbumin plasma level and binding capacity in protein-calorie malnutrition

The measurement of thyroxine-bindung prealbumin (TBPA) in protein-calorie malnutrition by two different techniques leads to the recognition of an unexpected discrepancy. Whereas TBPA plasma levels as measured by immunodiffusion are markedly decreased to 29.3% of the normal, those recorded by maximal binding capacity (TBPAcap) are characterized by a wide dispersion. In the control group, the ratio between TBPAcap and TBPA plasma level is 9.1. In the malnourished group on admission, the same ratio is 30.0. The possibility of a qualitative effect in the tetrameric TBPA structure, with the binding of additional thyroxine (T4) molecules on the secondary binding sites, has been investigated. This hypothesis has been discarded by Scatchard plot studies. The normal 1:1 molar ratio between TBPA and T4 is unaffected in protein-deficient patients. Discrepant results obtained for TBPAcap and TBPA levels appear to be the consequence of low plasma protein levels, leading to an artifact in the electrophoretic method.