The fetal thyroid gland is increasingly able to synthesize thyroid hormones from the second trimester of pregnancy. Abnormal maternal thyroid function in early pregnancy may disturb fetal brain development. A recent large study from Denmark indicates that severe maternal thyroid dysfunction in early pregnancy may adversely affect child intelligence.
How do exposures during pregnancy programme the fetus to later development of disease? This question has been the focus of much research for decades. Most pregnant women are aware of the risks related to alcohol consumption and smoking, but numerous other risk factors have been considered. A recent large Danish study on maternal thyroid function in early pregnancy shows that severe lack of maternal thyroid hormones may affect a child intelligence.
“The study was performed within the Danish National Birth Cohort, which recruited around 100,000 pregnant women in Denmark from 1997 to 2003. We evaluated maternal thyroid function by the measurement of thyroid hormone parameters in blood samples collected during the early pregnancy and stored in the Danish National Biobank. Using this design, we were able to identify women with undiagnosed and untreated thyroid dysfunction in the early pregnancy,” explains Stine Linding Andersen, MD, PhD and research fellow at Aalborg University Hospital.
Undiagnosed thyroid disorders
The thyroid gland synthesizes the thyroid hormones triiodothyronine (T3) and thyroxine (T4) and it is in turn regulated by the thyroid-stimulating hormone (TSH), which is secreted by the anterior pituitary gland. Thyroid hormones are important developmental factors and involved in the regulation of growth and activity in many parts of the body. In addition, they play a pivotal role in early brain development.
The fetal thyroid gland is increasingly able to synthesize thyroid hormones from the 12th week of pregnancy and thyroid hormones in the fetus are of maternal origin in the very early pregnancy. Evidence suggests that abnormal maternal thyroid function in pregnancy may disrupt fetal brain development, and an increased risk of neurodevelopmental disorders and low intelligence in the child may follow.
The blood samples used for the present study were collected in early pregnancy, median week 9 of pregnancy, and the researchers measured TSH and T4 in all the samples. Of the 1153 participants, 145 had abnormal thyroid function in early pregnancy, which was undiagnosed and untreated in the majority of cases.
“The study showed that one in eight children were born to mothers with some degree of thyroid dysfunction in early pregnancy. When the child turned 5 years, extensive psychological testing was performed including intelligence quotient (IQ) testing. Results showed that children born to mothers with severe thyroid dysfunction in the early pregnancy had lower IQ. Importantly, no deviation in IQ was seen among children born to mothers with mild aberrations in thyroid function.”
One of the challenges in this type of study is that many different exposures coincide during pregnancy and multiple influences on fetal development are possible.
Stine Linding Andersen comments that: “Our study accounted for other factors such as maternal IQ, smoking habits and alcohol consumption. Still, a number of risk factors that we do not know of or had no information on may also be at play. An important strength of our study is that we could measure maternal thyroid function in stored blood samples after the children were tested at 5 years of age. Thus, no information on exposure was available at the time of testing. This means that the exposure could not influence the outcome.”
Severe thyroid disorders
The two major types of thyroid dysfunction include hypo- and hyperthyroidism. In hypothyroidism, the thyroid gland produces insufficient amounts of thyroid hormone and TSH is elevated. In hyperthyroidism, the production of thyroid hormone is excessive and TSH is low.
“Our study showed that maternal TSH above 10 mIU/l was associated with lower child IQ. This finding is in accordance with previous studies showing that severe hypothyroidism during pregnancy may adversely affect fetal brain development. A major uncertainty has been on mild abnormalities and notably, we found that smaller deviations in maternal thyroid function did not associate with child IQ.”
The treatment of choice for thyroid dysfunction in pregnant women is medical treatment, but it remains unknown and debatable, which thyroid abnormalities to search for and when to treat.
Pregnant women in Denmark are not routinely tested for thyroid dysfunction. This study contributes to several studies that investigate the impact of maternal thyroid dysfunction in pregnancy on pregnancy outcomes and child development.
“Maternal thyroid function in early pregnancy and neuropsychological performance of the child at 5 years of age” has been published in the Journal of Clinical Endocrinology and Metabolism. The Novo Nordisk Foundation supports the Danish National Biobank, which stored the blood samples used in the study.