Variations in neonatal blood thyroxine concentrations appear to be largely attributable to fetal environmental factors, according to recent findings.
In the twin study, Nitash Zwaveling-Soonawala, MD, of Emma Children’s Hospital in Amsterdam, and colleagues reviewed record linkage from two databases: the Dutch neonatal congenital hypothyroidism screening, the results of which were stored at the National Institute for Public Health and Environment, and the Netherlands Twin Register.
The researchers identified 1,264 monozygotic and 2,566 dizygotic twin pairs (7,623) from the Netherlands Twin Register database born between 2006 and 2011. For these twins, information collected through a survey of the mothers included the following: maternal age at birth, gestational age, sex, birth weight, birth order and zygosity.
Blood thyroxine concentrations were measured in the participants on average on day 5 after birth. The researchers used structural equation modeling to establish maximum likelihood estimates of thyroxine score variance of the following influences: additive genetic factors, common environment shared by twins, and environmental factors not shared by twins. These estimates were determined for live-birth full-term twins, preterm twins and very preterm twins.
The researchers found that in the full-term group, additive genetic factors were responsible for 40% of the variance in boys and 31% of the variance in girls. Nonshared environmental factors comprised the remainder of the variation in thyroxine among the full-term group (33% for boys; 47% for girls).
In the preterm group, 34% of the variance in thyroxine levels among boys was attributable to genetic factors whereas none of the differences among girls were explained by genetic factors. Shared environmental factors accounts for 31% of variations in preterm boys and 57% in preterm girls and nonshared environmental factors accounted for 35% of variation in preterm boys and 43% in preterm girls.
In the very preterm group, genetics had no significant influence on thyroxine variations; shared environment accounted for 23% of variation in boys and 49% in girls, and nonshared environment accounted for 45% of variation in boys and 39% in girls.
According to the researchers, these findings may have implications in better understanding thyroxine set point.
“Since we analyzed neonatal screening results, this emphasizes the importance of the fetal environment for the postnatal [thyroxine] concentration,” the researchers wrote. “This suggests that the mechanism underlying [thyroxine] set point determination may lie in epigenetic modifications taking place in the fetal period. Heritability for [thyroxine] set point diminished, at least for girls, most likely due to the major environmental influences of immaturity and illness in very preterm infants.” – by Jennifer Byrne
Disclosure: The researchers report no relevant financial disclosures.