Experts are challenging claims that water fluoridation is linked to higher than expected rates of hypothyroidism.
A new study published in the Journal of Epidemiology & Community Health suggests water fluoridation — at rates used in New Zealand — is linked to 30 per cent higher than expected rates of underactive thyroid (hypothyroidism) in England.
The authors looked at the prevalence of underactive thyroid, as diagnosed by family doctors, and levels of fluoride in the drinking water supply provided by the Drinking Water Inspectorate.
In areas with fluoride levels above 0.7 mg/l, they found higher than expected rates of hypothyroidism than in areas with levels below this dilution.
In New Zealand, the Ministry of Health recommends fluoride levels in drinking water be adjusted to between 0.7 and 1.0 mg/L to benefit oral health.
The Science Media Centre collected the following expert commentary. Feel free to use these quotes in your reporting. If you would like to contact a New Zealand expert, please contact the SMC (04 499 5476; smc@sciencemediacentre.co.nz).
Dr David Crum, CEO, New Zealand Dental Association, comments:
“The paper appears to have a major flaw in that it fails to highlight that iodine deficiency is a major cause of hypothyroidism and that the two areas in the UK that have water fluoridation are also the areas that have high levels of iodine deficiency. In New Zealand we fortunately iodise our table salt so the situation here is different picture altogether.
“Peckham has misquoted the European Union Scientific Committee on Health and Environmental Risks. The truth is, that Committee actually stated, ‘Human studies do not suggest adverse thyroid effects at realistic human exposures to fluoride’.
“A report released last year titled ‘Health effects of water fluoridation: a review of the scientific evidence’ published on behalf of the Royal Society of New Zealand and the Office of the Prime Minister’s Chief Science Advisor strongly supports the findings of the review of the scientific evidence into the health effects of water fluoridation.
“The report strongly reaffirms the NZDA’s position that community water fluoridation is a safe, effective and cost efficient way of reducing dental caries and reducing health inequalities.”
“The NZ Dental Association and the NZ Medical Association along with the Ministry of Health, the Australian and American Dental Associations, the World Health Organisation and the World Dental Federation all support community water fluoridation.”
From the UK SMC
Dr Sandra White, Director of Dental Public Health at Public Health England, comments:
“Public Health England regularly reviews the evidence base for water fluoridation. The totality of evidence, accumulated over decades of research, tells us that water fluoridation is a safe and effective public health measure, and shows no association with reduced thyroid function. PHE’s own assessment of water fluoridation programmes in England found evidence of lower tooth decay rates in children living in fluoridated compared to non-fluoridated areas, and greater reductions among those living in the most deprived areas.”
Prof David Coggon, Professor of Occupational and Environmental Medicine, University of Southampton, comments:
“As epidemiological evidence goes, this is about as weak as it gets. Essentially the researchers have shown that after limited adjustment for demographic differences, there are somewhat higher rates of hypothyroidism (which can result from a number of different diseases) in four areas of England that have higher concentrations of fluoride in drinking water. It is quite possible that the observed association is a consequence of other ways in which the areas with higher fluoride differ from the rest of the country. There are substantially more rigorous epidemiological methods by which the research team could have tested their idea”
[This post was updated to correct an misplaced decimal point; the lower limit of Ministry of Health water fluoridation recommendations was erroneously stated as .07 mg/mL]