Direct assay of free thyroxine in the newborn

Several reagent sets are available, which allow a direct assay without dialysis of the free, bioavailable part of total thyroxine in plasma. A methodological investigation showed that the solid phase (coated tubes) kit from Clinical Assay gives reproducible results in plasma volumes of 50 microliter. Plasma samples from 95 newborns have been assayed at the 4th to 5th day of life. The mean concentration for free thyroxine of 62 infants born at term was 17 pmol/l with a log 2 SD range of 10-28 pmol/l. This value is similar to the normal value in adults (15.1 pmol/l, log 2 SD range 8.3-27.4 pmol/l). This is in contrast to the total T4 concentration: mean value for newborns 182 nmol/l (normal range 2 SD log 130-250 nmol/l) compared to a mean of 90 nmol/l (normal range 2 SD log 51-157) for adults. Mean free T4 concentration was 11.5 pmol/l in 18 premature infants (gestational ages 34-35 weeks: mean 8.7 pmol/l; gestational ages 36-37 weeks: mean 15.3 pmol/l). The lowest concentrations were found in infants with a respiratory distress syndrome: mean 8.9 pmol/l. TSH did not exceed 10 mU/l. It is concluded that the method used allows a direct estimation of free T4 in small plasma samples and allows a rapid decision in suspected thyroid disorders in infants.