BindingCharacteristicsof ThyroxinBindingGlobulinin Serumof Normal, Pregnant,and SeverelyHIEuthyroidPatients

Serum from normal, pregnant, and severely ill patientswas stripped of endogenousthyroidhormonesand diluted1100fold in barbital buffer. We then used it to study the binding characteristicsof thyroxin binding globulin (TBG), noting significant differences in binding capacities among the groups. The mean (± SD) trilodothyronine/thyroxinratio for bindingcapacity was 18 ±4 for normal subjects.The ratio was significantlyincreased in pregnant patients, 21 ±4 (p <0.05), and significantlylower in severely ill patients,12 ±4 (p <0.05). When serum was dilutedbefore assay, to give a uniform TBG concentrationamong groups, these apparent differences in binding characteristics were eliminated. It therefore is unlikelythat differentmolecularspeciesof TBG accountfor the variationsin bindingcharacteristicsin these clinical states. Apparently, the distributionof thyroxin and triiodothyronineamong the binding sites on TBG changes with variationsin TBG concentration.This may explain the discrepanciesobserved in the concentrationsof free thyroid hormonesas estimated by variousmethodologies.