Normal ranges of T4 screening values in low birthweight infants.

Thyroxine (T4) screening values in infants of low birthweight in relation to birthweight and gestational age are reported. There were 86 healthy infants of low birthweight (group 1), and 29 preterm infants with respiratory distress syndrome (group 2). All the group 2 infants and 36% of those in group 1 had a T4 screening value below the cut-off point (-2.1 SD). In group 1 there was a significant increase in T4 with birthweight at a given gestational age, as well as with gestational age at a given birthweight. In group 2 there was also a significant increase in T4 values in relation to birthweight and gestational age, but it could not be ascertained whether this increase existed at a given gestational age or birthweight. A statistical model giving normal ranges of T4 for both groups of infants is presented, which, if applied to low birthweight infants, makes it possible to estimate the effect of low birthweight on T4 screening values, provided the birthweight and gestational age are known. In this manner the sensitivity of screening for congenital hypothyroidism is enhanced and the recall rate reduced.