https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6843260/
"Our finding that thyroid cancer incidence rates are associated with the use of city water was unexpected and merits examination in other locations with a mix of city and well water use."
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"Our finding for city water use raises questions about factors associated with city water that could influence TC (Thyroid Cancer) risk. City water differs from well water in several respects; the most obvious are that city water is chlorinated and fluoridated. Data on the effects of chlorine on the thyroid are limited. However, studies with human volunteers indicate a lack of significant changes in thyroid hormone levels in adults consuming chlorinated water at a concentrations of 2 and 20 parts per million (mg/L) [37,38]. Conversely, there is evidence that fluoride may cause thyroid dysfunction [39]. Specifically, experimental studies in animals and observational studies in humans have reported alterations in thyroid hormone levels (e.g., T4, T3) and/or an increase in thyroid stimulating hormone (TSH) in individuals consuming fluoridated water [40,41,42]. Increases in TSH thus could promote the growth of latent TCs [43,44]. During the time period 1992–2006, the percentage of North Dakotans receiving city water that was “optimally fluoridated” (defined by the CDC as fluoride levels of 0.7–1.2 ppm) was > 96% (vs. 69.2% for the U.S. overall) [45]. In this regard, significant upregulation of TSH was reported among consumers of fluoridated water with levels as low as 0.5 mg/L [0.5 ppm] [46]."
"Our finding that city water use is associated with higher TC risk was unexpected and should be interpreted cautiously. However, it is consistent with several reports implicating fluoride as a cause of thyroid dysfunction and thus merits further investigation."