Fluoride exposure and hypothyroidism in a Canadian pregnancy cohort.

Hall M, Lanphear BP, Chevrier J, Hornung R, Green R, Goodman C, Ayotte P, Angeles Martinez-Mier E, Zoeller RT, Till C. Science of the Total Environment 2023 Apr 15;869:161149. doi: 10.1016/j.scitotenv.2022.161149

Fluoride is a mineral found naturally in most water sources. At low concentrations, fluoride in drinking water prevents cavities and improves oral health. About one-third of Canadians have fluoride added to their public water supply to protect teeth from decay. Thyroid hormones play an important role in many aspects of metabolism (how our bodies break down food into energy) and development, and can be disrupted by environmental toxicants. Researchers at York University conducted a study to investigate the relationship between fluoride and thyroid hormone levels during pregnancy. Building upon previous findings of a relationship between fluoride and children’s IQ, researchers also investigated the associations between maternal hypothyroidism and children’s IQ.

The researchers used data from the MIREC study. Fluoride was measured in women’s urine samples during pregnancy and in drinking water. Fluoride intake was estimated by combining the drinking water data and questionnaire data on the type of beverages women consumed during pregnancy. The association between fluoride and thyroid hormone levels was determined using statistical models. The association between maternal hypothyroidism and child IQ was analyzed using statistical model in a subsample of 439 mother-child pairs.

The researchers found that a 0.5 mg/L increase in drinking-water fluoride concentration was associated with 65% increased odds of primary hypothyroidism; 0.5 mg/L is roughly the difference in exposure level between a fluoridated and non-fluoridated community. No associations were seen between urinary fluoride or fluoride intake and hypothyroidism. Compared to children of women with normal thyroid levels, those born to women with primary hypothyroidism had lower IQ scores. This association was stronger in boys than girls.

Fluoride in drinking water was associated with increased risk of hypothyroidism in pregnant women. These findings provide further evidence of the potential impact of fluoride on thyroid function and, in turn, on children’s neurodevelopment.

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