Study linking fluoride, hypothyroidism criticized

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A study linking fluoride with hypothyroidism contained "myriad flaws" and used "highly selective and biased" evidence, according to a commentary in the British Dental Journal. The author of the opinion piece chided the lead researcher for abandoning his scientific objectivity to reinforce his opposition to fluoridation, saying the paper's conclusions should be dismissed.

The commentary takes issue with a study published earlier this year, which concluded that water fluoridation above a certain level is linked to 30% higher than expected rates of hypothyroidism in England (Journal of Epidemiology & Community Health, February 24, 2015). The study's lead author is Stephen Peckham, the director and a professor of health policy at the University of Kent Centre for Health Service Studies in Canterbury, U.K.

Michael Foley, BDS, MPH, director, Brisbane Dental Hospital, Australia.Michael Foley, BDS, MPH, director, Brisbane Dental Hospital, Australia.

In his opinion piece, Michael Foley, BDS, MPH, director of the Brisbane Dental Hospital in Australia, refutes the study for attributing a causal association between exposure and outcomes while distorting and misrepresenting the findings of several studies about fluoride's salutary effects (Br Dent J, November 13, 2015). Dr. Foley lectures at the University of Queensland and Griffith University dental schools in epidemiology and research methods, biostatistics, and water fluoridation.

Dr. Foley acknowledged Peckham's right to argue against water fluoridation.

"But in this paper his opposition to fluoridation overrode his academic responsibility to present his findings objectively," Dr. Foley told "[The] review of the academic literature was highly selective and biased, and their understanding of the limitations of the study was poor. The study failed to consider a host of possible confounders that most likely would have invalidated their conclusions."

Correlation is not causation

Although Peckham said he used a cross-sectional study design in his research, Dr. Foley asserted it was more of an ecological study.

"A cross-sectional study usually attempts to correlate exposure and outcome variables recorded from sampled individuals, whereas an ecological study derives data for either the exposure or outcome variable or both from population data," Dr. Foley wrote in an email to "Attributing a causal association between exposure and outcome variables in ecological studies is difficult, as they are particularly susceptible to bias and confounding. The ecological fallacy, by which inferences about individuals can be incorrectly made based on findings from population data, is well known."

Misleading references

Stephen Peckham, professor of health policy, University of Kent in the U.K.Stephen Peckham, professor of health policy, University of Kent in the U.K.

Peckham and colleagues wrote that the "effects of fluoride on the thyroid have long been observed" in their paper.

But Dr. Foley noted that assertion relied on many tenuous references: a brief mention in a study where "other researchers" reported that "fluoride is a thyroid inhibitor" (Journal of Dental Medicine, 1961, Vol. 16, pp. 190-199), personal communications with a U.S. Public Health Service researcher, and a 1954 paper that studied delayed tooth eruption in rats following removal of the pituitary gland.

"The evidence supporting Peckham et al's statement is extremely weak," Dr. Foley noted.

Peckham also cited findings that he said linked fluoride with goiters, but Dr. Foley pointed out the citation referred to patients with hyperthyroidism, and "did not find that fluoride was linked to goiter" (Journal of Clinical Endocrinology & Metabolism, October 1958, Vol. 18:10, pp. 1102-1110). That study found that hyperthyroidism occurred only occasionally from "massive" doses of fluoride. That situation, Dr. Foley noted, is "clearly not comparable to community water fluoridation."

In contrast, the British Thyroid Association in 2006 endorsed the conclusion of several international reviews: "There is no evidence that fluoride is responsible for any disorder of the thyroid."

Dr. Foley added: "Iodine intake could certainly confound any statistical association between fluoridation status and hypothyroidism. So why wasn't this considered by the authors? Peckham et al show little understanding of confounding factors, and have made only a token attempt at considering their impact."

While Peckham's study raised questions about the safety of community fluoridation, Dr. Foley pointed out that out of the three references supporting the statement, two "make no such alarmist recommendations," and one is Peckham's own paper (Scientific World Journal, February 26, 2014).

Studies supporting fluoridation ignored

Peckham also cited a 2006 National Research Council (NRC) report to support his contention that water fluoridation is associated with thyroid dysfunction. But after the report was issued, the U.S. Centers for Disease Control and Prevention (CDC) reiterated its support for fluoridation.

“The authors ... focus on a small number of poor-quality studies that reinforce their own views, while ignoring contradictory evidence from much stronger studies and reviews.”
— Michael Foley, BDS, MPH

"The CDC considers comprehensive reviews by the NRC and other systematic scientific studies in its recommendation that community water fluoridation is a safe, effective, and inexpensive method to reduce tooth decay among populations with access to community water systems," the agency stated. "Water fluoridation should be continued in communities currently fluoridating and extended to those without fluoridation."

Years of research by myriad public health agencies -- including the CDC, U.S. Food and Drug Administration, ADA, World Health Organization, National Cancer Institute, U.S. Public Health Service -- and nonpartisan groups such as the Pew Research Center have shown that fluoridation is a safe and effective way to prevent caries, especially among children and underserved populations who have limited access to dental care. The CDC considers water fluoridation one of the greatest public health achievements in the 20th century.

"The authors show a disturbing tendency to focus on a small number of poor-quality studies that reinforce their own views, while ignoring contradictory evidence from much stronger studies and reviews," Dr. Foley concluded in his opinion piece. "Peckham et al should have heeded the adage 'correlation is not causation' before coming to a conclusion at odds with a large body of reputable evidence from around the world. In my opinion, the paper's conclusions can and should be dismissed."

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