Thyroid system immaturities in very low birth weight premature infants

Continuing advances in the care of premature infants has contributed to the increased survival of very low birth weight premature infants. These infants are characterized by a variety of organ and physiological systems immaturities predisposing to deficiencies of postnatal adaptation and a high prevalence of neonatal morbidities. These morbidities have a major impact on postnatal mental and neurological outcomes. Thyroid hormones play a critical role in central nervous system development and function, and thyroid system immaturities as well as morbidity-related thyroid dysfunction (the nonthyroidal illness syndrome) contribute to the transient hypothyroxinemia of premature infants (THOP). Several studies have demonstrated a correlation of THOP with subsequent low IQ and neurologic sequelae in very low birth weight premature infants, and there is suggestive evidence that thyroid hormone supplementation in very low birth weight infants can improve mental outcome. Here, we review normal fetal thyroid system development and the system immaturities contributing to THOP and predisposing to nonthyroidal illness in very low birth weight infants.