Fluoride exposure may increase the risk of hypothyroidism during pregnancy

New research from the University of York has found that exposure to fluoride through drinking water may increase the risk of hypothyroidism in pregnant women. In a smaller subset of participants, the researchers found lower IQ scores among boys whose mothers had been diagnosed with hypothyroidism compared to boys whose mothers had normal thyroid levels.

Previous research conducted by the same lab in York found a link between maternal fluoride exposure during pregnancy and lower IQ in boys, and this new study may explain these earlier findings, said York neuropsychology doctoral student Meaghan Hall, lead author. of this latest study.

“We know from the literature that there is an association between high levels of fluoride exposure and thyroid dysfunction, and there is also an established association between untreated hypothyroidism in pregnancy and adverse outcomes in children,” says Hall. “Our latest study may provide a possible mechanism for linking lower IQs in boys born to mothers with higher fluoride exposure and is part of a growing body of evidence suggesting that prenatal fluoride exposure may be associated with poorer cognitive outcomes for children.”

The study, published today in Science of the Total Environment, followed more than 1,500 women enrolled in the Maternal Infant Research on Environmental Chemicals (MIREC) study, an ongoing, multi-year study led by Health Canada to investigate the impact of environmental chemicals on vulnerable populations. including pregnant people and infants. Women were recruited from 10 cities across Canada, seven of which contained drinking water containing fluoride. The researchers only studied women who reported drinking tap water during pregnancy. The women were followed during pregnancy and their children were also followed after birth through early childhood.

In the study, an increase of half a milligram per liter in the fluoride content of drinking water, which may not sound like much but is roughly the difference in exposure level between a fluoridated and non-fluoridated community, was associated with a 1.65 increase in chance of being diagnosed or meeting the criteria for hypothyroidism during pregnancy.

“This translates to a 65 percent increase in risk,” said Christine Till, Hall’s supervising professor, senior author of the study and a clinical neuropsychologist who heads the Till Lab at the Faculty of Health, which studies how different environmental exposures affect children’s health. “The findings are concerning because hypothyroidism is a known cause of brain disorders in children,” says Till.

Hall and Till hope that policymakers will consider this new research when evaluating the safety of water fluoridation in the community.

Previous research on the link between fluoride exposure and hypothyroidism has mostly been done with laboratory animals and children and adults living in parts of the world where fluoride levels are naturally high. Fluoride’s ability to suppress the thyroid has been known since the 1930s, when it was used to treat an overactive thyroid, also known as hyperthyroidism.

The mechanism by which fluoride may interfere with thyroid function isn’t fully understood, Hall says, though she says it may interfere with certain enzymes and the absorption of iodine, which is critical for thyroid hormone production.

Women are generally more at risk of developing hypothyroidism, a condition in which the body does not produce enough thyroid hormones and can lead to symptoms such as fatigue, weight gain, and depression. During pregnancy, the demands placed on the thyroid system increase significantly, especially in the first trimester when the fetus is solely dependent on maternal thyroid hormones.

The researchers measured exposure to fluoride from tap water and other dietary sources such as black tea, which is naturally high in fluoride. The researchers also measured urine fluoride levels and found no association with hypothyroidism. The researchers say that fluoride levels in tap water may be a more reliable indicator of long-term fluoride exposure than urine levels, which may correlate better with short-term exposure, they say.

About four in ten Canadians and seven in ten Americans with public water supplies have fluoridated drinking water. Fluoride compounds are added to tap water to reduce cavities on a population level and are a major source of fluoride exposure.

Reference: Zaal M, Lanphear B, Chevrier J, et al. Fluoride exposure and hypothyroidism in a Canadian pregnancy cohort. Sci Total Enviro. Published online February 9, 2023:161149. doi:10.1016/j.scitotenv.2022.161149

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