Low levels of thyroid hormone during pregnancy may contribute to the development of attention deficit hyperactivity disorder in the child, new research suggests.
The study found that children born to mothers with low thyroid hormone levels during the first trimester of pregnancy had a 28% increased risk of being diagnosed with ADHD later.
Thyroid hormones play an important role in the growth and development of the fetal brain, especially during the first trimester of pregnancy, the researchers said.
"The thyroid is important in pregnancy and can have long-term impacts," said study lead author Morgan Peltier. He's an associate professor in the departments of clinical obstetrics, gynecology and reproductive medicine at NYU Winthrop Hospital in Mineola, N.Y.
"These findings highlight the greater need for prenatal care," Peltier added.
ADHD is a neurodevelopmental disorder that affects more than 9% of U.S. children, according to the researchers. The condition leads to difficulty paying attention, impulsiveness and hyperactive behavior.
A number of genes are suspected to be involved in ADHD. Many of those genes are regulated by thyroid hormones, the team noted.
For the study, the researchers looked at records from Kaiser Permanente Southern California hospitals. These included data on nearly 330,000 children born between 2000 and 2016. Information on the children's health is collected until they reach 17.
Almost 17,000 children in this group were diagnosed with ADHD. The children were all evaluated for ADHD using the same criteria.
Almost 10,000 expectant moms were diagnosed with low thyroid hormone levels during pregnancy.
In addition to finding an overall increased rate of ADHD in children born to mothers who had low thyroid levels, they found a significant racial difference. Hispanic children whose mothers had low thyroid hormone levels during pregnancy had a 45% increased risk of ADHD. White children had a 22% increased risk.
Peltier said it's not clear from the data in this study why the effect of low thyroid hormones was stronger for Hispanic children.
The researchers also noted the effect of low thyroid hormones was more significant in boys than in girls. The study only found an association between thyroid levels and ADHD, rather than a cause-and-effect link.
Dr. Michael Cackovic is a maternal fetal medicine specialist at the Ohio State University Medical Center in Columbus, and was not part of the study. He said it highlights how important it is to "optimize women's health before pregnancy if you have the opportunity."
Cackovic said thyroid hormone screening isn't currently done in all women during pregnancy.
Symptoms of low thyroid include tiring easily, feeling cold, constipation, dry skin, being forgetful and feeling depressed, according to the American Thyroid Association.
If you have symptoms of low thyroid (hypothyroid), Cackovic said you should bring them up with your doctor. However, many of the symptoms of low thyroid -- such as fatigue or constipation -- are similar problems that occur in pregnancy.
Cackovic said more research would be needed to see if it makes sense to screen all women early in pregnancy. He said it would be important to know if screening and then treating women for low thyroid hormone levels makes a difference in ADHD rates.
ADHD isn't the only neurodevelopmental condition that may be linked to low thyroid hormones. The same researchers previously published a study in Nature that found an association between the development of autism in children and mothers with low thyroid hormone levels during pregnancy. They plan to study whether or not low thyroid hormones play a role in other neurodevelopmental disorders.
The study was published Oct. 21 in the American Journal of Perinatology.
Learn more about ADHD risk factors from the U.S. National Institute of Mental Health.