Circulating 3,3', 5'-triiodothyronine (reverse T3) in the human newborn

Area of Science:

• Endocrinology
• Neonatal Physiology
• Thyroid Hormone Metabolism

Background:

• Thyroid hormones, including thyroxine (T4), triiodothyronine (T3), and reverse triiodothyronine (rT3), play critical roles in neonatal development.
• Understanding thyroid hormone dynamics in newborns is essential for assessing physiological adaptation and identifying potential endocrine disturbances.

Purpose of the Study:

• To investigate the concentrations and trends of serum rT3, T3, and T4 in healthy full-term newborns during the early postnatal period.
• To compare neonatal thyroid hormone profiles with those of normal adults.
• To explore the relationship between thyroid-stimulating hormone (TSH) and rT3 levels in neonates and adults.

Main Methods:

• Measurement of serum rT3, T3, and T4 concentrations in cord blood and venous samples from newborns (birth to 30 days).
• Comparison of neonatal hormone levels with those of adult control groups.
• Assessment of neonatal rT3 response to exogenous TSH and correlation with T3 and T4 levels.

Main Results:

• Newborns exhibited significantly higher serum rT3 and T4, but lower T3, compared to adults.
• Serum rT3 levels in neonates did not significantly increase with the early postnatal TSH surge and showed limited response to exogenous TSH.
• Neonatal rT3 levels gradually decreased, becoming comparable to adult levels by 9-11 days of life, while T3 and T4 remained elevated compared to adults.

Conclusions:

• Serum rT3 concentrations in neonates are modulated by factors beyond TSH.
• The high neonatal rT3 levels observed shortly after birth decrease significantly within the first two weeks of life.
• Thyroid hormone profiles in healthy term newborns undergo rapid adaptation, reaching adult-like patterns within the first month.