Fluoride exposure and indicators of thyroid functioning in the Canadian population: implications for community water fluoridation
Barberio AM, Hosein FS, Quiñonez C, McLaren L — Journal of Epidemiology and Community Health
The Fluoride Science editorial board appraised a key fluoride thyroid research study finding no association between fluoride and hypothyroidism indicators in Canada.
WHAT THE RESEARCH FOUND
The authors conducted this cross-sectional study using data from the latest (2009-2013) Canadian Health Measures Survey (CHMS) to examine the association between fluoride exposure and thyroid outcomes.
Both thyroid outcome and fluoride exposure were measured at individual level as follows:
Fluoride exposure measures
- Fluoride level in spot urine sample
- Fluoride concentration in primary drinking water (mg/L) + self-report on residential history (at least 3 years of consecutive residence)
- Self-reported use of fluoride containing toothpaste and or mouthwash as well as history of fluoride treatment at dental office
Thyroid outcome measures
- Self-reported diagnosis of thyroid condition (yes/no)
- Serum thyroid stimulating hormone (TSH) level (low/normal/high)
No significant association was found between fluoride exposure measured in urine and tap water samples and self-reported diagnosis of a thyroid condition or altered (low or high) TSH levels. Fluoride exposure, in a time and place where multiple sources of fluoride including community water fluoridation exist, is not associated with impaired thyroid functioning in a representative sample of the Canadian population.
Canada has fluoridation program guidelines that are similar to the US, and the findings are relevant to the US and other countries with similar populations and CWF schemes.
LEVEL OF RIGOR
- A – Strong methodology and unbiased, appeared in peer-reviewed in respected science journal
- B – Strong methodology and unbiased, not in peer-reviewed journal
- C – Weak methodology and/or biased
- F – Not a scientific finding
SUPPORT FROM OTHER STUDIES
- High – All the peer-reviewed research to date support these findings, and a significant amount of research has been done in this area.
- Medium – Most, but not all, peer-reviewed research to date support these findings, and a significant amount of research has been done in this area.
- Low – Not a lot of research has been done in this area, or some, but not most, other peer-reviewed research supports these findings.
- Not Supported – No other studies support this study’s conclusions.
- Contradicted – Most studies contradict this study’s conclusions.
The association between fluoride exposure and thyroid outcomes was examined using the individual-level data collected from the nationally representative sample of Canadian and adjusted for potential demographic confounders (age, gender, household education and income).
In addition to self-reported data of thyroid condition diagnosis and indicators of fluoride exposures along with the sample of tap water to determine fluoride concentration of subjects’ primary drinking water, the authors used biomarkers of contemporary fluoride exposure (urinary fluoride level) and thyroid functions (serum TSH, thyroid stimulating hormone).
Data from the latest Canadian national health surveillance program (2009-2013), which implements extensive data validation and quality control measures
The use of self-reported thyroid condition diagnosis is subject to misclassification. Spot urine samples do not provide cumulative measures of fluoride exposure over time.
Cross-sectional study design, thus causation could not be discerned.
Potential confounders such as iodine intake (although Canada has adopted mandatory iodisation of all food-grade salt since 1949 and reportedly has adequate population iodine status), smoking, and family history of thyroid disease were not adjusted.
RELEVANCE AND VALIDITY
As a previous study on this topic by Peckham and colleagues reported an ecological association between fluoride and hypothyroidism (both exposure and outcome measured at medical practice-level in England), the findings of this study, which used data of exposure and outcomes measured at the individual-level, offers a design that overcomes the concern that an ecological fallacy is at play. Although each method of measuring fluoride exposure has some underlying limitations—misclassification due to fluctuation of spot urine sample; reporting/recall bias in self-reported use of fluoride products/fluoride treatment; unknown amount of actual fluoride exposure from primary drinking water—the use of multiple sources of information is a good strategy to identify individual-level fluoride exposure.
The overall prevalence of hypothyroidism or hyperthyroidism at the population-level is low (less than 5%) in Canada, thus the raw number of subjects who had altered serum TSH level or self-reported diagnosis of thyroid condition was small (nearly 95% of subjects had normal TSH level or no diagnosis of thyroid condition).
The study provides sound individual-level evidence that fluoride exposure levels experienced by the general population do not confer risk for disrupted thyroid function.