Study Linking Fluoridation to Hypothyroidism Criticized

Laird Harrison

December 22, 2015

A study published earlier this year in the Journal of Epidemiology and Community linking community water fluoridation to hypothyroidism continues to spark controversy and attract widespread publicity.

As previously reported by Medscape Medical News, the study suggested that areas of England with fluoridated community water had increased rates of hypothyroidism.

On the basis of this link and some previous research on fluoridation and thyroid function, the study authors conclude that "public dental health interventions should stop interventions reliant on ingested fluoride and switch to topical fluoride-based and non-fluoride-based intervention."

However, since the study's publication, critics have spoken out vehemently.

"There were glaring errors in the paper," said Michael Foley, BDSc, MPH, director of Brisbane Dental Hospital in Australia. "It just gobsmacked me how it could have been published."

Dr Foley published a point-by-point critique of the study's findings online November 13 and in the November issue of the British Dental Journal ( Br Dent J. 2015;219:429-431).

"There is no reputable evidence that community water fluoridation causes hypothyroidism," Dr Foley, who is a spokesperson for the Australian Dental Association, told Medscape Medical News.

The Journal of Epidemiology and Community Health also published commentaries in March ( J Epidemiol Community Health. 2015;69:616) and July ( J Epidemiol Community Health. 2015;69:617-618) that are critical of the study.

Despite the criticisms, prominent media organizations including the BBC, Newsweek, and the Chicago Tribune have reported on the study results. Public Radio International's "Living on Earth" described its conclusions at length in an audio clip without including opposing comments from health officials.

Others pointed out that the study's conclusion contradicts positions of leading public health bodies around the world, including the World Health Organization, the United Kingdom's Medical Research Council, Health Canada, Australia's National Health and Medical Research Council, and the Institute of Medicine in the United States.

The Centers for Disease Control and Prevention has called community water fluoridation one of the Ten Great Public Health Achievements in the 20th Century, and the US Department of Health and Human Services' Healthy People project has set as a goal for 2020 increasing the proportion of people served by fluoridated water from 72.4% to 79.6%.

Standing by the Findings

But the study's lead author, Stephen Peckham, BSc, MA, a professor of health policy at the University of Kent and the London School of Hygiene and Tropical Medicine in the United Kingdom, stands by his findings. "Some of the conclusions might be overstated, but I don't think that makes a difference to the actual analysis," he told Medscape Medical News.

Dr Peckham said he got interested in water fluoridation when it was proposed for Southampton in the United Kingdom, where he lives.

"I started to look at the evidence and found that perhaps the evidence for water fluoridation was not has clear cut as it was being presented," he said.

He joined the campaign against fluoridation that eventually succeeded in stopping the initiative in Southampton.

Later, after a colleague noted locations with elevated hypothyroidism around England, he began documenting an association with water fluoridation. About 10% of people in England live in areas where fluoride is added to water.

Relying on data kept by individual practitioners, Dr Peckham and colleagues estimated that the odds of a practice recording high levels of hypothyroidism was 1.4 times higher (odds ratio, 1.371; 95% confidence interval, 1.120 - 1.679) in areas with maximum fluoride levels, from higher than 0.3 mg/L to 0.7 mg/L, and 1.6 times higher in areas with maximum fluoride in excess of 0.7 mg/L (odds ratio, 1.621; 95% confidence interval, 1.379 - 1.904) than it was for practices in areas with maximum fluoride levels of 0.3 mg/L or less.

Dr Peckham and colleagues also cite earlier research suggesting that high levels of fluoride can disrupt thyroid function; in particular, a 2006 review by the National Research Council of the US Environmental Protection Agency's standards for fluoride in drinking water.

That review concluded that "several lines of information indicate an effect of fluoride exposure on thyroid function. is difficult to predict exactly what effects on thyroid function are likely at what concentration of fluoride exposure and under what circumstances."

An "Appalling Paper"

Not only is the National Research Council report equivocal, other reviews cited in Dr Peckham's paper also do not support his conclusions, says Dr Foley. For example, the European Commission's Scientific Committee on Health and Environmental Risks concludes that, "A systematic evaluation of the human studies does not suggest a potential thyroid effect at realistic exposures to fluoride."

As for Dr Peckham's finding of a correlation between fluoridation and hypothyroidism in England, Dr Foley said that it did not adequately take into consideration the potential that geographical variation in iodine intake could confound the results.

Dr Peckham and colleagues acknowledge the importance of iodine to thyroid function, but they write that "the major source of iodine in the UK is from the diet and it is unlikely that there are significant differences between people residing in fluoridated and non-fluoridated areas."

Yet the studies Dr Peckham and colleagues cite do not show that iodine levels are uniform from one part of England to another, Dr Foley argued.

"I think Peckham should be ashamed of himself," Dr Foley said. "It's an appalling paper."

"It's a Bit of a Mine Field"

Dr Peckham, who is preparing a written response to the some of these critiques, has not budged from his bottom line. The evidence for a benefit is not strong enough to support community water fluoridation, and the evidence for harm is not weak enough to be dismissed, he said.

"I think on that basis, I would recommend that oral health switch to noningested fluoride," said Dr Peckham.

He cited a study published in June in the Cochrane Database of Systematic Reviews. The study concluded that "water fluoridation is effective at reducing levels of tooth decay among children," but that the "the results are based predominantly on old studies and may not be applicable today."

In the meantime, with both sides entrenched in their positions, research is difficult, he said. "It's a bit of a mine field."

Dr Peckham disclosed that he was involved in a campaign in Southampton to prevent the fluoridation of drinking water. The other authors and Dr Foley have disclosed no relevant financial relationships.

J Epidemiol Community Health. Published online February 24, 2015. Abstract


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