Last updated: March 29, 2016
FLUORIDE AND THYROID: LACK OF EVIDENCE OF A ROLE IN ENDOCRINE DISORDERS
Fluoride is a naturally occurring chemical element and a member of the halogen family sharing chemical similarities with other halogens including iodine. The thyroid produces hormones that contain iodine such as thyroxine (T4) and triiodothyronine (T3) that regulate metabolisms such as growth and development. Epidemiological observations of mottled enamel, now known as severe fluorosis, and concurrent goiter in human populations, as well as clinical observations of an anti-thyroid effect among some patients who received therapeutic doses of fluoride for demineralizing disorders or hyperthyroid in the past, have interested scientists better understand how fluoride interacts with the thyroid gland and other endocrine system.1-3
The vast majority of available studies, animal or human studies, concerning the effect of fluoride on thyroid functions had numerous limitations and do not consistently point in one direction on fluoride and thyroid.1,4 Many studies used a very small number of study subjects, failed to control critical confounders such as iodine and/or selenium intake or other substances in water (i.e. calcium), measured the exposures ecologically, or were conducted outside of US, where there was high prevalence of iodine deficiency, under-nutrition, and goiter, as well as naturally occurring fluoride.1,5-9 Some studies showed the potential interaction of fluoride with thyroid functions, such as altered T3 or T4 concentrations and/or elevated thyroid stimulating hormone (TSH), which is a large protein hormone secreted from the pituitary gland, at higher doses of fluoride exposure5-8 and when iodine was deficient.9 However, these studies suffered from many of the above-mentioned limitations in their designs, were at high risk for bias, and thus the findings are not generalizable to US populations.
Peckham and colleagues analyzed data from general medical practices in England and published a study in 2015 showing an association between the practice-level prevalence of hypothyroidism and fluoride level in drinking water. While the authors made the conclusion that fluoride exposure should be considered as a contributing factor to hypothyroidism, this ecological study, which is affected by several flaws in study design, provided no evidence of causal relationship between water fluoridation and hypothyroidism.10-14
Potential mechanisms of action for the reported associations between fluoride intake and biological outcomes in thyroid (i.e. goiter or alteration of thyroid hormones) are poorly understood due to insufficient scientific evidence and the complexity of the thyroid system.1 In general, pathways by which thyroid functions could be disrupted or altered by foreign chemicals include 1) direct action on a hormone-receptor protein complex or 2) a direct action on a specific protein that controls some aspect of hormone delivery to the right place at the right time.15 Several sets of results from human studies suggest fluoride’s action on deiodinase activity, but other mechanisms of action are also possible.1 More research is recommended especially in light of declined iodine intake in the US, existence of environmental endocrine disruptors, as well as increased thyroid disease and disorders worldwide.
In summary, there is no compelling scientific evidence that support optimally fluoridated communities in the US to alter the current practice for the purpose of preventing adverse health effect, including thyroid disease.
SYSTEMATIC REVIEWS THAT DISCUSS ENDOCRINE DISORDERS, FLUORIDE AND THYROID
MCDONAGH ET AL. 20004
Three cross-sectional studies that evaluated the link between fluoride level in drinking water and goiter prevalence met the inclusion criteria. The findings were mixed, and the authors concluded no association between water fluoridation and the prevalence of goiter.
NRC REPORT 20061
The review committee examined a wide-range of animal and human studies published in English and non-English languages to assess the adequacy of the U.S. Environmental Protection Agency’s current maximum-contaminant level goal (MCLG) of 4 mg/L for fluoride. They acknowledged limitations in available evidence, cautiously concluded that evidence is suggestive of the link between fluoride exposure in humans and elevated TSH concentrations, increased goiter prevalence, and altered T4 and T3 concentrations, and recommended further research.
AAFP REPORT 201316
Three studies, two cross-sectional studies that examined the correlation between community water fluoride levels and rates of goiters and one small clinical trial using sodium fluoride to treat osteoporosis, met the inclusion criteria. They concluded that no credible evidence was found of any adverse effects of community water fluoridation on thyroid functions in humans.
SUTTON ET AL. 201517
This review conducted by Ireland Department of Health was to evaluate any impact, positive or negative, on the general health of those exposed to community water fluoridation at its current levels in Ireland (0.4-1.5ppm). The authors agreed on conclusions from systematic reviews by McDonagh et al. and determined the Peckham fluoride and thyroid study to be low quality that offers no relevant evidence.
- National Research Council. Fluoride in Drinking Water. National Academies Press. Washington, DC. 2006
- Galletti PM, Joyet G. Effect of fluorine on thyroidal iodine metabolism in hyperthyroidism. J Clin Endocr. 1958;18:1102
- Davis PJ. Fluoride therapy and the thyroid gland. JAMA. 1966;196(13):1159
- McDonagh MS, Whiting PF, Wilson PM et al. Systematic review of water fluoridation. BMJ. 2000 Oct 7;321(7265):855-9
- Bachinskii PP, Gutsalenko OA, Naryzhniuk ND, Sidora VD, Shliakhta AL. Action of the body fluorine of healthy persons and thyroidopathy patients on the function of hypophyseal-thyroid the system (Original article written in Russian). Probl Endokrinol. 1985;31(6):25-9
- Susheela AK, Bhatnagar M, Vig K, and Mondal NK. Excess fluoride ingestion and thyroid hormone derangements in children living in Delhi, India. Fluoride. 2005;38(2):98-108
- Mikhailets ND, Balabolkin MI, Rakitin VA, Danilov IP. Thyroid function during prolonged exposure to fluorides (Original article written in Russian). Probl Endokrinol. 1996;42(1):6-9
- Singh N, Verma KG, Verma P, Sidhu GK, Sachdeva S. A comparative study of fluoride ingestion levels, serum thyroid hormone & TSH level derangements, dental fluorosis status among school children from endemic and non-endemic fluorosis areas. Springerplus. 2014 Jan 3;3:7
- Lin FF et a. The relationship of a low-iodine and high-fluoride environment to subclinical cretinism in Xinjiang. Iodine Deficiency Newsletter. Vol. 7/ No.3 1991
- Peckam 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;69(7):619-24
- Newton JN, Young N, Verne J, Morris J. Water fluoridation and hypothyroidism: results of this study need much more cautious interpretation. J Epidemiol Community Health. 2015;69(7):617-8
- Warren JJ, Saraiva MC. No evidence supports the claim that water fluoridation causes hypothyroidism. J Evid Based Dent Pract. 2015;15(3):137-9
- Foley M. Fluoridation and hypothyroidism- a commentary on Peckham et al. Br Dent J. 2015;19(9):429-31
- Segal P. Is fluoridated drinking water associated with a higher prevalence of hypothyroidism? Clinical Thyroidology for the Public. Vol.8(6). June 2015. Available at http://www.thyroid.org/wp-content/uploads/publications/ctfp/ct_public_v86.pdf
- World Health Organization. State of the Science of Endocrine Disrupting Chemicals 2012. Available at https://apps.who.int/iris/handle/10665/78102
- The American Academy of Family Physicians. Systematic Evidence Review of Community Water Fluoridation. May 2013. Available at http://www.aafp.org/about/policies/all/fluoride.html
- Sutton M, Kiersey R, Farragher L, Long J. Health effects of water fluoridation: An evidence review 2015. Available at https://www.hrb.ie/fileadmin/publications_files/Health_Effects_of_Water_Fluoridation.pdf
Appraisals & commentaries
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