Thyroid function in healthy normal, low birthweight and preterm infants

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Abstract
To delineate more precisely the role of gestational age, weight at birth and thyroid status at birth on the postnatal changes in thyroid hormone levels, serum T4, T3, TSH and in some cases FT3I were measured at birth and at 3–4 h, 24–30 h, 6–9 days and 13–20 days. Subjects studied were healthy appropriate-for-date (AFD) and small-for-date (SFD) term neonates and healthy AFD and SFD preterm children. At birth T4 and T3 are related to both gestational age and weight with T4 and T3 showing lower values in preterm and SFD term neonates than in AFD term children. After birth T4 and T3 concentrations show a better correlation with gestational age than with weight at birth. For TSH no correlation was found at birth, a positive correlation at 24–30 h, no correlation at 6–9 days and a negative correlation at 13–20 days both with gestational age and weight at birth. In term and close-to-term infants (36 weeks) individual T4 levels at 6–7 days show a colse relationship with those at birth; in the younger children (34 and 35 weeks) lower T4 values are found, despite equal cord blood values. The individual cord blood FT3I/TSH values correlate well with those at 6–7 days of age. It is concluded that after birth all children have changing T4 and T3 values, but the pattern and level are influenced by the maturity of the child and its thyroid status at birth measured by T4 and by the FT3I/TSH ratio. Weight at birth influences T4 at birth, but during the 1st week the weight of the child influences the T4 and T3 levels in only a minor way.