Are high fluoride levels in drinking water associated with hypothyroidism prevalence in England?

Background and aims

Studies have suggested that there is a possible link between fluoride exposure and thyroid function.

The aim of this study was to examine whether there are differences in the prevalence of

hypothyroidism between populations in England living in areas with high and low levels of fluoride in

drinking water.

Methods

We used a cross-sectional study design using secondary data to develop a linear regression model of

predictive factors for hypothyroidism prevalence at practice level using fluoride levels in drinking

water, hypothyroidism prevalence data, the proportion of registered patients that were female and

the proportion of older patients (aged 55+ and 65+). We compared the hypothyroidism prevalence

of GP practices in areas with fluoride levels ?0.7mg/l to those with fluoride levels <0.3mg/l. We also

analysed for between group differences of hypothyroidism prevalence for two metropolitan areas:

West Midlands Metropolitan County, a wholly fluoridated area; and Greater Manchester, a nonfluoridated

area.

Results

Of the 8,020 practices in England, prevalence data for 7,946 practices (99.1%) were used to develop

the linear regression model. The model accounted for 52% of the observed variance and was

significant. It predicted that hypothyroidism prevalence will increase as fluoride levels increase after

controlling for age and gender. The comparison of high and low fluoride areas showed a significantly

higher prevalence of hypothyroidism in GP practices in the high fluoride areas. The comparison of

West Midlands Metropolitan County and Greater Manchester showed a higher mean practice level

prevalence of hypothyroidism in the West Midlands, although the result was not statistically

significant.

Conclusions

The findings of the study show that higher levels of fluoride in drinking water are associated with a

higher prevalence of hypothyroidism. This observation provides the first population level

confirmation of the findings of previous studies suggesting that ingestion of fluoride suppresses

thyroid function.