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

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3890436/#:~:text=The%20fluoride%2C%20the%20negative%20ion,proportion%20to%20its%20atomic%20weight.

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Iodine Deficiency Disorders (IDD) and fluorosis are the two most prevalent endemic diseases which coexist in certain regions in India (Hetzel et al. 1990).
As early as 1928, Stocks (1928) observed that children consuming well water in the village of Somerset, England exhibited both goiter and mottled enamel (dental fluorosis).
Some years later, Wilson (1941) found dental fluorosis (DF) associated with goiter and cretinism among children living in areas of Punjab where fluoride was recognized geologically to be significantly high.

As it is already known that fluoride is more electronegative than iodine, it easily displaces iodine within the body, thereby affects the functioning of thyroid gland. Fluoride has been known to have shown gross as well as biochemical changes within the body of an individual which included deranged thyroid hormonal level with in the body.
The production of thyroid hormones is regulated by a negative feedback mechanism, i.e., when the pituitary gland senses a drop in FT3 levels in circulation, it releases more TSH to stimulate the thyroid gland which in turn accelerates the production of the thyroid hormone T4, now considered a “pro-hormone”. The major source of circulating T3 is from peripheral deiodination of T4 and not from thyroid secretion. The enzymes which catalyze deiodination are called iodothyronine deiodinases and fluoride is known to interfere with the activity of the deiodinases.

Conclusion
The results of this study question the validity of the fluoridation of drinking water, milk, fruit juices, and salt by public health authorities and also the step taken to prevent ill effects of excess fluorine and iodine deficiencies in endemic fluorosis areas.
The children with dental fluorosis living in endemic fluorosis areas may not have a frank thyroid disease due to excessive fluorine consumption but they do show thyroid disease leading to many health effect hence they require special care and attention.