The hypothalamic-pituitary-thyroid axis in preterm infants; changes in the first 24 hours of postnatal life

Area of Science:

• Neonatal endocrinology
• Perinatal physiology
• Thyroid hormone metabolism

Background:

• Thyroid hormones are crucial for neonatal adaptation after birth.
• Preterm infants, especially those with very low gestational age, may exhibit altered thyroid function.
• The hypothalamic-pituitary-thyroid (HPT) axis undergoes significant changes in the transition from fetal to neonatal life.

Purpose of the Study:

• To measure key thyroid hormones and binding proteins in preterm infants during the first 24 hours of life.
• To compare thyroid hormone profiles across different gestational age groups (24-27, 28-30, and 31-34 weeks).
• To characterize the HPT axis response to birth in extremely preterm infants.

Main Methods:

• Serum samples collected at cord, 1, 7, and 24 hours postpartum.
• Assays performed for T4, free T4, TSH, T3, reverse T3 (rT3), T4 sulfate, and thyroxine-binding globulin.
• Infants stratified into three gestational age groups: 24-27 wk, 28-30 wk, and 31-34 wk.

Main Results:

• The TSH surge at 1 hour was markedly attenuated in the 24-27 wk group compared to older groups.
• T4 levels declined in the most immature group but increased in more mature groups over 24 hours.
• Free T4 and T3 showed minimal increases in the immature group, contrasting with sustained increases in older preterm infants.
• Higher cord levels of rT3 and T4 sulfate were observed in the most immature infants.

Conclusions:

• Infants born between 24-27 weeks gestation exhibit a severely attenuated or failed HPT axis response to delivery.
• This distinct endocrine response differentiates the extremely preterm group from more mature preterm infants.
• The findings highlight the unique physiological challenges and adaptations in the earliest preterm neonates.