T3 is correlated with intelligence and fluoride exposure is inversely correlated with intelligence.

[learnedhelpless]

"Fluoride exposure was inversely related to IQ scores (B = −1.587; 95% CI: −2.607, −0.568 for water fluoride and B = −1.214; 95% CI: −1.987, −0.442 for urinary fluoride).... Higher TT3, FT3 were related to the increased odds of children having high normal intelligence "

Thyroid function, intelligence, and low-moderate fluoride exposure among Chinese school-age children

 

[Rinse & rePeat]

“However, a statistically significant difference was observed in the urinary fluoride levels (P 0.000). Reduction in intelligence was observed with an increased water fluoride level (P 0.000). The urinary fluoride level was a significant predictor for intelligence (P 0.000).

Conclusion:​

Children in endemic areas of fluorosis are at risk for impaired development of intelligence.”

Effect of fluoride exposure on the intelligence of school children in Madhya Pradesh, India

To assess the relationship between exposure to different drinking water fluoride levels and children's intelligence in Madhya Pradesh state, India.This cross-sectional study was conducted among 12-year-old school children of Madhya Pradesh state, India. ...

www.ncbi.nlm.nih.gov

 

[Mauritio]

"Fluoride exposure was inversely related to IQ scores (B = −1.587; 95% CI: −2.607, −0.568 for water fluoride and B = −1.214; 95% CI: −1.987, −0.442 for urinary fluoride).... Higher TT3, FT3 were related to the increased odds of children having high normal intelligence "

Thyroid function, intelligence, and low-moderate fluoride exposure among Chinese school-age children

Interesting.
I remember watching a documentary with a similar topic, it was about why we're getting dumber, showing a european doctor who treated pregnant women with iodine, which resulted in lower cases of autism and I think also higher intelligence. I guess the iodine displaced flouride.
It was a very interesting documentary and I posted it on the forum although the video is not available anymore.