Perinatal Factors Affecting Thyroid Hormone Status in Extreme Preterm Infants

Pragmatic criteria are required for defining transient hypothyroxinemia and to permit entry to clinical trials of thyroxine substitution of only those extreme preterm infants who are hypothyroxinemic. The purpose of this article is to suggest that transient hypothyroxinemia is defined by postnatal serum T4 levels, which are cord levels corrected to an equivalent gestational age had the fetuses remained in utero, and that those levels are adjusted for the significant prenatal and intrapartum factors. Lowered serum FT4 levels are not a consistent pathognomonic feature of transient hypothyroxinemia as postnatal FT4 levels in this large series of preterm infants are within or above the cord values of equivalent gestational age, irrespective of severity of illness. Although serum T3 and thyroid-stimulating hormone levels do not contribute to the diagnosis of transient hypothyroxinemia, measurement of their levels is nevertheless required for trial monitoring involving thyroxine substitution to avoid inadvertent suppression of the developing hypothalamic–pituitary–thyroid axis by excess T4 substitution.