The authors of the study in the Journal of Epidemiology and Community Health say their findings raise concerns about the use of fluoride in drinking water as a safe and effective public health measure for improving dental health.
The study of 98.9% of GP practices in England found that high rates of underactive thyroid (hypothyroidism) were most likely in areas with the highest fluoride levels in tap water.
Public Health England says it has reviewed the evidence surrounding fluoridation and concluded that it is safe and effective.
Artificial water fluoridation has been used as a way of preventing tooth decay in some areas of the UK for several decades.
In England, around 6 million people, or 10% of the population, live in areas with a naturally or artificially fluoridated water supply containing 1 mg fluoride per litre of drinking water.
The researchers from the University of Kent looked at the 2012 levels of fluoride in the drinking water supply, using data provided by the Drinking Water Inspectorate for individual postcodes. These were then matched against cases of hypothyroidism diagnosed by GPs in these areas.
In areas with fluoride levels above 0.7 mg/l, they found higher than expected rates of hypothyroidism than in areas with lower levels, and even in practices in areas where fluoride levels were in excess of 0.3 mg/l, high rates of hypothyroidism were 30% more likely.
The researchers also compared 2 large urban areas: The West Midlands, which is supplied with fluoridated drinking water, and Greater Manchester, which is not. In this analysis the difference was striking, with GP practices in the West Midlands nearly twice as likely to report high rates of hypothyroidism as those in Greater Manchester.
Underactive thyroid gland
Symptoms of an underactive thyroid include:
- Weight gain
- Being sensitive to the cold
- Dry skin and hair
- Muscle aches
The researchers say because the study made use of existing data, no definitive conclusions can be drawn about cause and effect. They also point out that they were not able to take account of other sources of fluoride found in dental products and food and drink.
However, they say: “Consideration needs to be given to reducing fluoride exposure, and public dental health interventions should stop [those] reliant on ingested fluoride and switch to topical fluoride-based and non-fluoride-based interventions.”
‘Safe and effective’
Commenting on the research in a statement, Dr Sandra White, director of dental public health at Public Health England, says: “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.”
Research published last year in the British Dental Journal said that millions of pounds spent by the NHS in England each year on extracting rotten teeth could be saved if water fluoridation was extended to areas of the country with the highest levels of tooth decay.
Medically Reviewed by Dr Keith David Barnard