Normalized thyroxine as a screening test for hypothyroidism in full-term and preterm newborn babies

Thyroid function was assessed in full-term and preterm newborn babies by serum thyroxine (T4), normalized thyroxine (T4N) and thyroid-stimulating hormone (TSH) assays. At age 24 h, there was a significant difference in T4 and TSH values between the full-term and preterm groups; no such difference was found in the T4N values. By 21 days of age, the TSH values were still significantly higher in full-term babies compared with preterm ones, but the T4 values were similar. The T4, T4N and TSH values at 24 h in preterm newborns with respiratory distress syndrome were similar to those in normal preterm babies, and the changes in these values with age had no consistent pattern. In preterm babies with low 24-h T4 and T4N values, these two parameters increased with age, reaching normal adult values by 21 days. We concluded that T4N could serve as a useful thyroid function test in the newborn.