Perchlorate in drinking water during pregnancy and neonatal thyroid hormone levels in California.

OBJECTIVE To evaluate associations between maternal drinking water perchlorate exposure during pregnancy and newborn thyroid hormone levels. METHODS Elevation in thyroid stimulating hormone (TSH), which may reflect reduced thyroxin concentration, was assessed in 497,458 newborns in California in 1998. A total of 800 perchlorate water measurements were used to classify California communities as exposed (>5 μg/L) or unexposed. Results were stratified by age at TSH collection because of the normal postbirth TSH surge, and because water sources and perchlorate exposure can change soon after birth. RESULTS In TSH samples collected within 24 hours of birth, the odds ratio for a TSH greater than 25 μU/mL in exposed communities was 1.53 (P < 0.0001). After 24 hours, the odds ratio for a TSH more than the 95th percentile was 1.27 (P < 0.0001). CONCLUSION These findings suggest that perchlorate is associated with increased neonatal TSH levels.