Relationship between free T4 levels and postnatal steroid therapy in preterm infants

Background: Transient hypothyroxinemia is the most common thyroid dysfunction in preterm infants. Hypothalamic-pituitary-thyroid immaturity and non-thyroidal illness contribute to its etiology. The aim of the present study was therefore to determine the relationship between thyroid hormone status and early postnatal steroid therapy in preterm infants. Methods: A prospective study of premature infants born at < 28 weeks of gestation between July 2001 and June 2007 was conducted. Selective postnatal steroid (dexamethasone) therapy was used in lung disease treatment if the infants needed high mean airway pressure-assisted ventilation and supplemental oxygen at 2 weeks of age. Free T4 (FT4) and thyroid-stimulating hormone (TSH) levels were assessed at 2 weeks after birth. Blood samples in eight infants were available after starting steroid therapy. Infants receiving steroids (steroid (+); n = 8) were compared to those not receiving steroids (steroid (-); n = 73). Results: The demographic data were not significantly different between the two groups. The neonatal illnesses and drug use were also not significantly different between the groups. The steroid (+) group had significantly lower FT4 and TSH levels at 2 weeks after birth than the steroid (-) group. The increase in FT4 levels after steroid withdrawal was greater than that during the same period in the steroid (-) patients. Conclusion: Even if it cannot be excluded that reduced FT4 and TSH concentrations are caused by non-thyroidal illness, the present study suggests that postnatal steroid treatment reduces the FT4 and TSH levels in premature infants born at < 28 weeks of gestation.