Researchers say healthy thyroid functioning is crucial for fetal brain development and their latest research may explain earlier findings looking at fluoride and children’s IQ

TORONTO, Feb. 09, 2023 –  found that fluoride exposure via drinking water may increase the risk of hypothyroidism in pregnant women. In a smaller subset of participants, the researchers found lower IQ scores between boys whose mothers had been diagnosed with hypothyroidism compared to boys whose mothers had normal thyroid levels.

Earlier research done by the same lab at 91��ɫ found an association between maternal fluoride exposure in pregnancy and lower IQ in boys, and this new research may explain these earlier findings, according to 91��ɫ neuropsychology PhD student Meaghan Hall, lead author of this latest study.

“We know from the literature that there is a link between high levels of fluoride exposure and thyroid disruption and there’s also an established connection between untreated hypothyroidism in pregnancy and adverse outcomes in children,” says Hall. “Our latest study may provide a potential mechanism to explain links to lower IQs in boys born to mothers with higher fluoride exposure and is part of a growing body of evidence that suggests that prenatal fluoride exposure may be linked to worse cognitive outcomes for children.”

The study, published today in Science of the Total Environment, followed more than 1,500 women enrolled in the  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 have fluoridated drinking water. The researchers only studied women who reported drinking tap water in pregnancy. The women were followed throughout pregnancy and their children were also followed after birth into early childhood. 

In the study, a half-milligram-per litre increase in drinking-water fluoride levels, 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 odds of having a diagnosis or meeting criteria for hypothyroidism in pregnancy.

“This translates into a 65 per cent increase in risk,” says , Hall’s supervising professor, senior author on the study and a clinical neuropsychologist who heads up the Faculty of Health’s Till Lab, which studies how various environmental exposures affect children’s health. “The findings are concerning because hypothyroidism is a known cause of brain-based disorders in children,” says Till.

Hall and Till say they hope that policy makers will consider this new research when evaluating the safety of community water fluoridation. 

Previous research looking into the link between fluoride exposure and hypothyroidism has mostly been done with lab 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 overactive thyroid, also known as hyperthyroidism. 

The mechanism by which fluoride may interfere with thyroid function is not entirely clear, says Hall, although she says that it may interfere with certain enzymes and iodine absorption, which is critical for thyroid hormone production. 

Women in general are more at risk of developing hypothyroidism, a condition where the body does not synthesize enough thyroid hormones and can lead to symptoms such as fatigue, weight gain and depression. In pregnancy, the demands put on the thyroid system increase substantially, especially in the first trimester when the fetus is solely reliant 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 urinary fluoride levels and found no link with hypothyroidism. The researchers say fluoride levels in tap water may be a more reliable indicator of long-term fluoride exposure than urinary levels, which might better correlate with short-term exposure, they say.

Approximately four in 10 Canadians and seven out of 10 Americans on public water supplies have fluoridated drinking water. Fluoride compounds are added to tap water as a way to reduce cavities at the population level and this is a major source of fluoride exposure. 

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Emina Gamulin, 91��ɫ Media Relations, 437-217-6362, egamulin@yorku.ca

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TORONTO, October 10, 2018��– A new study led by 91��ɫ researchers has found that fluoride levels in urine are twice as high for pregnant women living in Canadian cities where fluoride is added to public drinking water as for those living in cities that do not add fluoride to public water supplies.

The study “Community Water Fluoridation and Urinary Fluoride Concentrations in a National Sample of Pregnant Women in Canada” was published today in .��It is the first study in North America to examine how fluoride in water contributes to urinary fluoride levels in pregnant women. The research was conducted as part of a larger study funded by the National Institute of Environmental Health Sciences, part of the National Institutes of Health (NIH) investigating whether early life exposure to fluoride affects the developing brain.

“We found that fluoride in drinking water was the major source of exposure for pregnant women living in Canada. Women living in fluoridated communities have two times the amount of fluoride in their urine as women living in non-fluoridated communities,” said Christine Till, an associate professor of Psychology in 91��ɫ’s Faculty of Health and lead author on the study.

The Maternal Infant Research on Environmental Chemicals (MIREC) study recruited 2,001 pregnant women between 2008 and 2011. The women lived in 10 large cities across Canada. Seven of the cities (Toronto, Hamilton, Ottawa, Sudbury, Halifax, Edmonton and Winnipeg) added fluoride to municipal water while three (Vancouver, Montreal and Kingston) did not.

Urine samples were collected during each trimester of pregnancy for over 1,500 women. Fluoride levels in municipal water treatment plants that provided water to each women’s home were obtained. Information about each woman’s demographics, lifestyle and medical history was also collected.

In addition to fluoridated water, sources of fluoride can include toothpastes, mouth rinses, as well as processed beverages and food, especially those made with fluoridated water. Beyond water, products such as tea have previously been found to have high concentrations of natural fluoride.

In this study, fluoride level in water was the main determinant of fluoride level in the women’s urine. Higher consumption of black tea was also correlated with higher levels of urinary fluoride in pregnant women.

The levels of fluoride among pregnant women living in fluoridated communities in Canada were similar with levels reported in a prior study of pregnant women living in Mexico City where fluoride is added to table salt.

“This finding is concerning because prenatal exposure to fluoride in the Mexican sample has been associated with lower IQ in children. New evidence published today in  also reported an association between higher levels of fluoride in pregnancy and inattentive behaviours among children in the same Mexican sample,” said Till.

The research team, including experts from Simon Fraser University, Université Laval, Indiana University, University of Montreal and Cincinnati Children’s Hospital, is investigating whether prenatal exposure to fluoride in Canadian children results in IQ deficits, similar to the Mexican study.

Fluoride has been added to public drinking water in Canadian and American communities since the 1940s as a means of preventing tooth decay.��Today, about 40 per cent of Canadians and 74 percent of the U.S. population on public water supplies receive fluoridated water.

91��ɫ champions new ways of thinking that drive teaching and research excellence. Our students receive the education they need to create big ideas that make an impact on the world. Meaningful and sometimes unexpected careers result from cross-disciplinary programming, innovative course design and diverse experiential learning opportunities. 91��ɫ students and graduates push limits, achieve goals and find solutions to the world’s most pressing social challenges, empowered by a strong community that opens minds. 91��ɫ U is an internationally recognized research university – our 11 faculties and 25 research centres have partnerships with 200+ leading universities worldwide. Located in Toronto, 91��ɫ is the third largest university in Canada, with a strong community of 53,000 students, 7,000 faculty and administrative staff, and more than 300,000 alumni.

91��ɫ U's fully bilingual Glendon Campus is home to Southern Ontario's Centre of Excellence for French Language and Bilingual Postsecondary Education.

Media Contacts:
Anjum Nayyar, 91��ɫ Media Relations, 416-736-2100 ext. 44543, anayyar@yorku.ca

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