Fluoride has two major differences that would complicate such a study: First, fluoride is a natural component of lots of drinking water (often at levels far higher than artificial fluoridation creates), while lead contamination in drinking water is rare and usually human-caused. Second, lead is known to be bad for one's health in any amount, while fluoride is only known to cause IQ drops above a certain dose.
> This systematic review and meta-analysis found inverse associations and a dose-response association between fluoride measurements in urine and drinking water and children’s IQ across the large multicountry epidemiological literature. There were limited data and uncertainty in the dose-response association between fluoride exposure and children’s IQ when fluoride exposure was estimated by drinking water alone at concentrations less than 1.5 mg/L.
CDC agreed with the 2006 National Research Council findings that fluorides “interfere with the function of the brain and body by direct and indirect means.” CDC KNEW fluoride affected the brain back in 2006, yet did nothing with this information! https://youtu.be/0A34FK2lTDs
You might find this meta-analysis interesting: https://jamanetwork.com/journals/jamapediatrics/fullarticle/... Part of that conclusion (note that water fluoridation in the US is recommended to a level of 0.7 mg/L):
> This systematic review and meta-analysis found inverse associations and a dose-response association between fluoride measurements in urine and drinking water and children’s IQ across the large multicountry epidemiological literature. There were limited data and uncertainty in the dose-response association between fluoride exposure and children’s IQ when fluoride exposure was estimated by drinking water alone at concentrations less than 1.5 mg/L.