That study looked at 27 studies in Asia and found only when crossing 2.5mg/L fluoride concentration did Thyroid issues present in a non-linear fashion.

> Dose-response curve showed no change in TSH concentrations in the lowest water fluoride exposure range, while the hormone levels started to linearly increase around 2.5 mg/L

The CDC recommends 0.7mg/L in water sources. Not a concern.

https://www.cdc.gov/fluoridation/about/community-water-fluor...