Serial blood T4 and TSH determinations in low birth weight infants. Influence of gestational age, birth weight and neonatal pathology on thyroid function.

Serial blood T4 and TSH determinations were performed on a total number of 352 low birth weight (LBW) infants: 210 healthy newborns and 142 infants affected by respiratory distress syndrome (RDS), sepsis, hyperbilirubinaemia, hypocalcemia, or hypoglycemia. The healthy infants have been divided into three groups in relation to gestational age (less than or equal to 33 wk, 34-36 wk, greater than or equal to 37 wk). Statistical evaluation among groups shows that reduced T4 concentration in LBW infants is strictly related to gestational age, while TSH failed to demonstrate any significant correlation. No significant influence of birth weight on T4 and TSH has been observed. The preterm newborns affected by RDS or sepsis showed a significant reduction of mean T4 concentrations up to 20th day of life when compared with healthy controls similar for gestational age and birth weight. TSH levels are not significantly different. No significant modifications of T4 and TSH have been found related to the other pathological conditions. The significant reduction of T4 and the high incidence of subjects with low T4 values observed in healthy and sick preterm population lend further support to the use of TSH determination for optimum screening of congenital hypothyroidism.