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Thyroid function in very preterm infants: influences of gestational age and disease
A G van Wassenaer1, J H Kok, F W Dekker
1Department of Neonatology, Academic Medical Center, University of Amsterdam, Emma Children's Hospital AMC, The Netherlands.
Insights
Thyroid hormone levels in preterm infants (<30 weeks gestation) are influenced by gestational age and illness. Younger infants and sick infants show distinct thyroid hormone patterns postnatally.
Area of Science:
- Neonatal endocrinology
- Pediatric thyroidology
Background:
- Postnatal thyroid function in preterm infants (<30 weeks gestational age) is not well understood.
- Immaturity and disease likely impact thyroid hormone levels in this vulnerable population.
Purpose of the Study:
- To investigate the influence of gestational age and disease on the time course of thyroid hormones in infants born <30 weeks gestation.
- To analyze serial measurements of thyroxine (T4), free T4 (FT4), triiodothyronine (T3), reverse T3 (rT3), TSH, and T4-binding globulin (TBG) during the first 8 postnatal weeks.
Main Methods:
- Serial measurements of thyroid hormones and TBG in 100 infants <30 weeks gestation.
- Infants were grouped by gestational age (25-28 vs. 28-30 weeks) and health status (sick vs. healthy).
Main Results:
- Thyroid hormone profiles (T4, FT4, T3, TSH, TBG) differed significantly between gestational age groups.
- T4 and FT4 decreased postnatally, with a deeper nadir on day 7 in younger infants.
- Disease significantly decreased thyroid hormone concentrations in the first week but led to higher levels after 3 weeks compared to healthy infants.
- Reverse T3 did not increase in sick infants and did not differ between gestational age groups.
Conclusions:
- The postnatal decrease in free T4 in very preterm infants is primarily influenced by gestational age, suggesting transient depletion of thyroid hormone reserves.
- Reverse T3 is not a reliable marker for nonthyroidal illness in very preterm infants.
Abstract:
It is not known how immaturity and disease influence postnatal thyroid function in infants <30 wk of gestational age. We performed serial measurements of plasma thyroxine (T4), free T4 (FT4), triiodothyronine (T3), reverse T3 (rT3), TSH, and T4-binding globulin (TBG) in 100 infants of <30 wk of gestation, during the first 8 postnatal weeks, to investigate the influences of disease and gestational age on the time course of thyroid hormones. One hundred infants were divided twice into two groups: 1) in a group of 25-28 and of 28-30 wk of gestation; and 2) in a sick and a healthy group, with similar gestational ages. The time course of T4, FT4, T3, TSH, and TBG, but not rT3 differed significantly (p < 0.005) between the gestational age groups. T4 and FT4 decreased to levels below the cord blood value with a deeper FT4 nadir on d 7 in the youngest group. Disease decreased T4, FT4, T3, TSH, and TBG concentrations especially during the 1st wk after birth (p < 0.005). However, the FT4 nadir on d 7 was similar in sick and healthy infants. After 3 wk, T4, FT4, T3, and TBG were higher in the sick group compared with the healthy group. rT3 levels were not increased in sick infants. We conclude that the extent of the FT4 decrease after birth in infants of <30 wk gestation is mainly influenced by gestational age and probably reflects a transient depletion of thyroidal hormone reserves. rT3 cannot be used as a marker of nonthyroidal illness in very preterm infants.