Comparisons between serum concentrations of thyroxine and thyroxine-binding proteins in samples simultaneously obtained from capillary, peripheral vein, central vein and aorta in newborn infants.

A total number of 40 newborn infants with various maturity were studied: 13 babies without perinatal events, 19 infants recovered from transient diseases, 6 infants with idiopathic respiratory distress syndrome and 2 infants with asphyxia indicating artificial ventilation. Comparisons were performed between serum concentrations of thyroxine (T4), thyroxine-binding globulin (TBG), prealbumin (TBPA) and albumin (Alb) in capillary versus peripheral vein, aorta versus central vein and, finally, in peripheral versus central veins. In healthy infants serum T4 concentrations in capillary blood and peripheral vein did not differ significantly. Although serum concentrations of thyroid hormone-binding proteins tended to be increased in aortic compared to central venous specimens no statistically significant differences appeared. In infants in good clinical conditions serum T4, TBG, TBPA, and Alb levels were 6--8% higher in peripheral than in central veins, possibly primarily due to a hemo-concentrating effect of venous stasis. Therefore, in evaluation of the thyroid variables in newborn infants the technique of blood sampling must be considered. In most infants with idiopathic respiratory distress syndrome and in one asphyxiated baby a remarkable tendency to a low serum TBG and T4 concentration in peripheral compared to central vein samples were observed.