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Science in the News

Fluoride Exposure and Thyroid Function in the Canadian Population

September 05, 2017 The thyroid is a gland that releases hormones which regulate development and metabolism. Thyroid underactivity (hypothyroidism) and overactivity (hyperthyroidism) are most commonly caused by autoimmune diseases, inflammation of the thyroid, and deficient or excessive iodine intake.1 Fluoride occurs naturally in all water and in food and beverages at varying levels.2 Community water fluoridation involves adjusting the drinking water to include the optimal amount of fluoride to reduce dental decay.2 It has been hypothesized that high levels of fluoride exposure affect thyroid function, although the mechanisms remain unclear and the effects inconsistent.3 To examine the association between fluoride exposure and thyroid conditions or functioning, a study evaluated pertinent data from the Canadian Health Measures Survey from the years 2009 through 2013.1

The Canadian Health Measures Survey collects health data on a sample of Canadians by both interview and clinical examinations.4 It is designed to be nationally representative of Canadians aged 3 to 79 years, excluding those who live in reserves or Aboriginal settlements, are full-time members of the Canadian Forces, or are institutionalized. To evaluate the association of fluoride exposure to thyroid function, this study used a combination of objective measures and survey responses.1 Contemporary fluoride exposure was estimated by measuring the fluoride content of spot urine samples. Interviews and public data were used to identify those who drank fluoridated tap water and lived in their current home for at least three years (for the 2009-10 sample), or who used fluoride containing products at home or ever received fluoride treatments from a dentist. Fluoride concentration of tap water samples were also collected for a subsample (2012-13 only). Levels of thyroid-stimulating hormone (TSH) and free thyroxine (T4) were quantified from blood samples collected from participants in 2012-13. Levels of TSH and T4 identified those with primary hypothyroidism. From 2009-13, participants were asked whether they had been diagnosed with a thyroid condition. Thyroid functioning was based on these three measurements: TSH level, primary hypothyroidism, or self-reported diagnosis of a thyroid condition. 

Participants had 33.55 µmol/L mean urinary fluoride in 2009-11, and 29.81 µmol/L in 2012-3. Slightly more than half or participants (55%) lived in areas with community water fluoridation. Impaired thyroid function was not common: 5.6% self-reported a thyroid condition, 93.2% had a normal level of TSH, and 0.5% had primary hypothyroidism. Individuals with low, normal, or high TSH levels had statistically similar mean urinary fluoride and fluoride concentration of tap water levels.1 After adjustment for age, sex, household income, and education, TSH continued to be unassociated with urinary fluoride or fluoride concentration in tap water. There were no significant differences in mean urinary fluoride or fluoride concentration of tap water between those with or without self-reported thyroid conditions. In multivariate logistic regression models, age, sex, and education were significantly associated with self-reported thyroid conditions, but not urinary fluoride or the fluoride concentration of tap water. In summary, the researchers did not detect any association between fluoride exposure and thyroid function.

There are strengths and limitations to this study. It draws from a nationally representative survey, so these findings are generalizable to the Canadian population and potentially other similar populations. It includes measurements and adjustment for potential confounders such as age or sex, although it lacked information on family history of thyroid disease. It includes multiple, individual-level measurement methods of fluoride exposure and thyroid function. Self-reported thyroid conditions are subject to recall bias. Urinary fluoride is subject to fluctuations, and as a marker of present or very recent exposure to fluoride, does not necessarily correlate with extended or prior exposure to fluoride. However, this study also assessed multiple sources of longer term fluoride exposure, such as drinking fluoridated tap water, using fluoride-containing products, or receiving fluoride treatments from a dentist.

This analysis of a nationally-drawn sample of Canadians (n=5,234) did not detect an association between levels of fluoride exposure and measures of impaired thyroid function. This is the first population-based analysis on the subject to use individual-level measures.

References

  1. Barberio AM, Hosein FS, Quiñonez C, McLaren L. Fluoride exposure and indicators of thyroid functioning in the Canadian population: implications for community water fluoridation. Journal of Epidemiology and Community Health. 2017. 
  2. Fluoridation Facts. American Dental Association Council on Access, Prevention and Interprofessional Relations. Accessed October 11 2015.
  3. Liu H, Zeng Q, Cui Y, Yu L, Zhao L, Hou C et al. The effects and underlying mechanism of excessive iodide on excessive fluoride-induced thyroid cytotoxicity. Environmental Toxicology and Pharmacology. 2014;38(1):332-40. doi:10.1016/j.etap.2014.06.008.
  4. Statistics Canada. Canadian Health Measures Survey (CHMS). 2014. Accessed 8/31/2017.

Prepared by: Center for Scientific Information, ADA Science Institute

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