Water fluoridation as a public health measure was introduced as a public health measure in America in the 1950s. The first public water fluoridation schemes in the UK began in the 1960s. Hypothyroidism affects women more than men (15 per 1,000 compared with 1 per 1,000 for men). Worldwide, iodine deficiency is a common cause of underactive thyroid although this is uncommon in the UK.
The aim of this study is to examine whether there are differences in prevalence of hypothyroidism between populations in fluoridated and non-fluoridated areas.
A cross-sectional study using secondary data and binary logistic models compared the hypothyroidism prevalence at general medical practice (GMP) level with fluoride levels in the drinking water. GMP were included if list size > 900 and Index of Multiple Deprivation (IMD) score was available. Practices were divided into two groups: those with low-to-medium hypothyroidism prevalence (lower two tertiles, 0.18: 3.57%), and high hypothyroidism prevalence (upper tertile, 3.58: 8.48%).
In the first model, all English data was included in the second model only practices from built-up areas in a non-fluoridated area (Greater Manchester) and fluoridated area (West Midlands) were included. Independent variables considered for inclusion in the model were:-
- average (mean) concentration of fluoride in drinking water (mg/L),
- maximum concentration of fluoride in drinking water (mg/L),
- practice level IMD scores,
- proportion (%) of the practice population aged 40 and over
- proportion (%) of the practice population that was female.
Mean and maximum fluoride concentrations were provided by the Drinking Water Inspectorate (DWI).
- For model 1; the adjusted odds ratio s for the upper tertile hypothyroidism prevalence according to fluoride levels in drinking water were:-
- Maximum fluoride >0.7 mg/L = 1.621 (95% CI; 1.379 to 1.904)
- Maximum fluoride >0.3, ≤0.7 mg/L = 1.371 (95% CI; 1.120 to 1.679)
- For model 2; The adjusted odds ratio for the prevalence of hypothyroidism in West midlands compared to Manchester = 1.935 ( 95% CI; 1.388 to 2.699)
The authors concluded:
We found that higher levels of fluoride in drinking water provide a useful contribution for predicting prevalence of hypothyroidism. We found that practices located in the West Midlands (a wholly fluoridated area) are nearly twice as likely to report high hypothyroidism prevalence in comparison to Greater Manchester (non-fluoridated area).
This cross-sectional data compared hypothyroidism and water fluoridation levels. The authors highlight some potential biases that could affect their results, water sampling frequency, inclination to request hypothyroidism test and GPs giving greater consideration to queried hypothyroidism in fluoridated areas. Apart from age, sex and practice IMD no other confounders were considered.
The 2000 York Review (McDonagh et al )* did not include any studies suggesting adverse effects of fluoridation on the thyroid gland although the 2006 NRC review of the EPA standards did note some endocrine effects with fluoride concentration in drinking water of 4mg/L and suggested further research.
This new study provides new information although this work needs to be repeated with further consideration of additional potential confounders before the validity of community water fluoridation as a public health measure can challenged.
* Note:– The York Review is currently being updated by the Cochrane Collaboration (Iheozor-Ejiofor et al 2013).
Peckham S, Lowery D, Spencer S. J. 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. Epidemiol Community Health Published Online First: [24 Feb 2015] doi:10.1136/ jech-2014-204971.
McDonagh M, Whiting P, Bradley M, Cooper J, Sutton A, Chestnutt I, et al. A Systematic Review of Community Water Fluoridation. NHS Centre for Reviews and Dissemination, University of York 2000.
National Research Council (NRC). Fluoride in drinking water: a scientific review of EPA’s standards. Washington DC: National Academies Press, 2006.
Iheozor-Ejiofor Z, O’Malley LA, Glenny AM, Macey R, Alam R, Tugwell P, Walsh T, Welch V, Worthington HV. Water fluoridation for the prevention of dental caries (Protocol). Cochrane Database of Systematic Reviews 2013, Issue 12. Art. No.: CD010856. DOI: 10.1002/14651858.CD010856.