Appraisal Endocrine Disorders

Fluoride hypothyroidism study suffers from ecological fallacy

Publication reviewed:

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

Peckham S, Lowery D, Spencer S — Journal of Epidemiology & Community Health

Dentist and public health specialist, Tanvi Dusane appraises problems in a fluoride hypothyroidism study prone to issues with the ecological fallacy.

Conceptual example of ecological fallacy a fluoride hypothyroidism study is at risk of


The authors conducted a large ecological study to evaluate the association of fluoride in drinking water with hypothyroidism in England. The units of analysis were general medical practices characterized by practice-level prevalence of hypothyroidism. These were assessed in relation to fluoride level in drinking water at the site of the medical practices. Mean and maximum concentration of fluoride in drinking water (mg/L) was provided by the Drinking Water Inspectorate. Hypothyroidism prevalence was obtained from the Quality of Outcomes data set. Covariates included in this study were practice-level IMD scores (a measure of social deprivation for a geographic area), proportion of the practice population ≥40 years of age and proportion that was female. The authors concluded that fluoride exposure is a contributing factor for hypothyroidism and suggested to stop dental public health interventions reliant on ingested fluoride based on this ecological study and despite the context of existing knowledge. The hypothesis generated from this study merits further study. Research that uses individual-level data and accounts for iodine intake would be useful. It is premature to raise public health concerns about the risk of hypothyroidism attributable to fluoride exposure based on this study.


  • A – Strong methodology and unbiased, appeared in peer-reviewed in respected science journal
  • B – Strong methodology and unbiased, not in peer-reviewed journal
  • C – Weak methodology and/or biased
  • F – Not a scientific finding


  • High – All the peer-reviewed research to date support these findings, and a significant amount of research has been done in this area.
  • Medium – Most, but not all, peer-reviewed research to date support these findings, and a significant amount of research has been done in this area.
  • Low – Not a lot of research has been done in this area, or some, but not most, other peer-reviewed research supports these findings.
  • Not Supported – No other studies support this study’s conclusions.
  • Contradicted – Most studies contradict this study’s conclusions.


This is the first large population-level assessment of the association between fluoride level in water and practice-level prevalence of hypothyroidism. The study has good measures for prevalence of hypothyroidism in medical practices.


This fluoride hypothyroidism research has important limitations in its design: This is an ecological study and the findings are at risk for the ecological fallacy. That is, findings may not be true at the level of the individual. Individuals with hypothyroidism may not have been exposed to the levels of fluoride in drinking water that were used in the study. Other potential confounding factors have not been taken into account (i.e…iodine intake, medical history, family history, smoking). The authors’ inaccurate interpretation of the findings from previous studies on fluoride in drinking water and thyroid function may have generated bias.


This is an ecological assessment using secondary data of private care practice-level prevalence of hypothyroidism in England and fluoride level in drinking water. The validity of this fluoride hypothyroidism study’s findings may suffer from ecological bias. Other limitations in study design involve lack of control for other potential confounders and lack of assessment of iodine intake.