Canadian adults who have higher levels of fluoride exposure and are deficient in iodine may be at an increased risk of underactive thyroid, according to a new study.
“Concern about exposure to fluoride disrupting thyroid function has been suggested in the past,” said Christine Till, the study’s senior author and associate professor at York University. “In fact, up until the 1960s, fluoride was prescribed as a suppressant for overactive thyroid. While our findings only establish an association, not cause-and-effect, they are important because they suggest a potential effect of fluoride on thyroid function at levels of exposure that are typical for Canadians.”
The study, “Fluoride exposure and thyroid function among adults living in Canada: Effect modification by iodine status”, published in Environment International, used population-based data from Cycle 3 of the Canadian Health Measures Survey (CHMS), which includes health-related data collected by Statistics Canada from a nationally representative sample of Canadians.
The study consisted of 6.9 million adults between the ages of 18 and 79 years. Adults who were diagnosed with a thyroid disorder, or who were on thyroid medication, as well as pregnant women, were excluded.
Exposure to fluoride comes from a variety of sources, though the major source of intake is from drinking water that is fluoridated to prevent tooth decay. Approximately 39 per cent of the population in Canada receives artificially fluoridated drinking water compared with about 74 per cent of the U.S. population.
The research team, based out of York University in Toronto, and the Department of Environmental Medicine and Public Health, Icahn School of Medicine at Mount Sinai in New York, tested whether higher levels of fluoride in urine are associated with disrupted thyroid function in adults living in Canada. Thyroid hormones help to regulate many vital bodily functions, including breathing and heart rate, digestive function, muscle strength, body temperature, and mood. They are also critical during pregnancy for fetal brain development.
Fluoride was measured in urine samples, which were then adjusted for dilution. Thyroid stimulating hormone (TSH) was measured in blood samples and used to assess thyroid function. The analyses also adjusted for other factors that could influence TSH level, including age, sex, body mass, as well as calcium levels in blood.
The association between urinary fluoride level and underactive thyroid function was not seen among the general population of adults, consistent with past studies. However, the researchers then tested whether the impact of fluoride exposure may be dependent upon having adequate iodine levels.
“Prior studies investigating fluoride exposure and thyroid function among adults did not consider iodine status – an important modifying factor,” said lead author, Ashley Malin, a former graduate student from the Department of Psychology at York University and now a post-doctoral fellow at the Department of Environmental Medicine and Public Health, Icahn School of Medicine at Mount Sinai in New York. “It was particularly important that we considered iodine status in this study because iodine deficiency is relatively common among adults in Canada.”
Iodine is essential for the production of thyroid hormone and iodine deficiency is believed to exacerbate thyroid-disrupting effects of certain chemicals, including fluoride. In the current study, almost 18 per cent of adults fell in the moderately-to-severely iodine deficient range.
The mechanism by which fluoride and iodine interact to affect thyroid function is not yet fully understood.
“Fluoride may interfere with enzymes that contribute to thyroid hormone synthesis and active transport of iodine into the thyroid,” Malin said. “Iodine may also facilitate bodily excretion of fluoride, thereby buffering the thyroid from its potential adverse effects.”
“If we can understand the reasons behind this association, we can then begin to develop preventive strategies to mitigate the risk,” said Till, who is also the principal investigator of a grant funded by the National Institutes of Health aimed at examining fluoride exposure in a large Canadian sample of pregnant women.