Historically, fluoride was used as a treatment for hyperthyroidism; however, it is not a treatment that is currently used. Fluoride is thought to be an antagonist of iodine, impacting upon the ability of body tissues, such as the thyroid gland, to utilise iodine. However, this is only important if iodine levels are low. Fluoride is commonly added to drinking water supplies as a public health intervention in industrialised countries. This is because fluoride is an important component of the defence against tooth decay, and carries significant benefits for oral health. But is there an association between fluoridated water supplies and increased rates of hypothyroidism?
Recently, an English observational study was published. This study compared the levels of fluoride in the water in different regions of London with the prevalence of diagnosed cases of hypothyroidism at GP practices in the same region.
The authors of the study claimed that there was an increase in the prevalence of hypothyroidism in areas with higher levels of fluoride in the water¹.
However, this study has been criticised in the media by experts for flaws in the study’s methodology². Firstly, the study employs a very simplistic design to investigate whether or not there is an association. The authors do not take into account the numerous other potential confounding factors, which may influence the results of the study. This type of study design makes it virtually impossible to prove a cause-and-effect relationship between fluoridated water supplies and hypothyroidism because there are so many other factors that can influence the rates of hypothyroidism, for example iodine intake, beyond just the levels of fluoride in the water.
Furthermore, the levels of fluoride in the water are at a very low level in the water supply. These levels are significantly below the amount of fluoride needed to have an inhibitory effect on the thyroid gland. Experts quoted in the article in The Conversation² are critical of how the authors of the paper exaggerate the potential effect of fluoride on the thyroid gland, and do not give sufficient recognition to the potential influence of iodine deficiency on the results.
Due to the flaws in the methodology of this study, the results of this study should be interpreted with a degree of scepticism. Further studies with a better design are required to determine if there is a cause and effect relationship between fluoride in the water and hypothyroidism.
Peckham S, Lowery D, Spencer S. Are fluoride levels in drinking water associated with hypothyroidism prevalence in England? A large observational study of GP practice data and fluoride levels in drinking water. J Epidemiol Community Health, 2015. Published online 24 February 2015. http://jech.bmj.com