Has perchlorate in drinking water increased the rate of congenital hypothyroidism?

Perchlorate, known to inhibit the human thyroid at doses above 200 mg/day, was detected in the drinking-water supplies of seven counties in California and Nevada at levels of 4 to 16 micrograms/L in 1997. The data from the neonatal screening programs of the state health departments in these two states were analyzed for any increased incidence of congenital hypothyroidism in those counties. County-specific, ethnicity-specific data for Nevada and California were obtained for 1996 and 1997. Within these seven counties, nearly 700,000 newborns had been screened. In all, 249 cases were identified, where 243 were expected, for an overall risk ratio of 1.0 (95% confidence interval, 0.9 to 1.2). The risk ratios for the individual counties ranged between 0.6 and 1.1. These data in this ecological analysis do not indicate an increase in the incidence of congenital hypothyroidism with the reported perchlorate levels.