Perchlorate contaminates important sources of drinking water in California and nationally. It inhibits the uptake of iodide into the thyroid gland, an essential part of the process of making thyroid hormone. Small changes is maternal thyroid hormone are associated with significant decreases in IQ. As well, small decreases in adult T4 levels are also associated with increased cardiovascular risk.
Perchlorate has been shown to block iodine uptake into the thyroid gland and cause a reduction in thyroid hormone production. In the past, evidence showed that this effect occurred only at very high doses of perchlorate. Recently however, several epidemiologic studies have provided evidence that lower levels of perchlorate commonly found in food and water sources in the U.S. might also impact thyroid hormone production. These studies suggest that these impacts primarily occur in certain susceptible populations. Earlier studies may have missed these effects because these groups were not specifically evaluated. Potential susceptible groups include pregnant women and their children, people with moderately low iodine intake, and those who consume foods with other iodine uptake blockers like thiocyanate. Given the large number of people in the US who are exposed to perchlorate and the important health effects associated with even minor reductions in thyroid hormone levels, the new evidence linking low levels of perchlorate to reductions in thyroid hormone could have profound public health implications.