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Developmental changes in pituitary-thyroid function in the human fetus and newborn
Insights
Thyroid hormone levels in newborns mature significantly with gestational age, indicating a changing feedback control system in premature infants. This maturation continues until two months of life.
Area of Science:
- Endocrinology
- Neonatal Physiology
- Developmental Biology
Background:
- The hypothalamic-pituitary-thyroid (HPT) axis undergoes significant development during gestation and early infancy.
- Understanding thyroid hormone regulation in premature infants is crucial for assessing their health and development.
Purpose of the Study:
- To assess the maturation of the HPT axis in premature and full-term infants.
- To correlate thyroid hormone concentrations with gestational age, sex, and birthweight.
- To compare neonatal thyroid hormone levels with those of older infants and adults.
Main Methods:
- Measurements of thyroxine (T4), free T4 (FT4), thyrotropin (TSH), and thyroxine-binding globulin (TBG) in cord serum.
- Correlation analysis with gestational age, sex, and birthweight.
- Comparison with data from 2-month-old infants and adults.
Main Results:
- Significant increases in T4, FT4, and TBG were observed with increasing gestational age from 26 to 33-35 weeks.
- After 34 weeks of gestation, these hormone levels did not vary significantly with gestational age.
- The FT4/TSH ratio increased with gestational age and was lower in cord blood compared to older infants and adults.
Conclusions:
- The set point for negative feedback control of TSH secretion changes between 26 weeks gestation and 2 months of life.
- Thyroid gland sensitivity to TSH stimulation appears to increase between 26 and 33 weeks gestation.
- These findings highlight the dynamic maturation of the HPT axis in early life.
Abstract:
Maturation of the hypothalamic pituitary thyroid axis as reflected in cord serum thyroid hormone concentrations was assessed in premature and full term infants born between 26 and 43 weeks gestation. Measurements of thyroxine (T4), free T4 (FT4), thyrotropin (TSH) and thyroxine binding globulin (TBG) in cord sera were correlated with gestational age, sex and birthweight and compared to similar measurements in well two month old infants and adults. There were significant increases in T4, FT4, and TBG with increasing gestational age (GA) between 26 and 33-35 weeks (P less than 0.001). After 34 weeks, none of these parameters varied with GA. When the infants were separated on the basis of sex the linear regression curves describing the relationships between hormone and TBG concentrations and GA were not different from the curves in the total population. The mean FT4/TSH ratio increased significantly with age throughout gestation (P less than 0.01) and was significantly lower in cord blood samples than in blood samples from the 2-month-old infants or the adults. The results suggest that the set point for negative feedback control of TSH secretion at the pituitary level is changing between 26 weeks GA and 2 months of life. Thyroid gland sensitivity to TSH stimulation also appears to be increasing between 26 and 33 weeks GA.