We have conducted a longitudinal epidemiologic study among pregnant women from three cities in northern Chile: Taltal with 114 microg/L, Chañaral with 6 microg/L, and Antofagasta with 0.5 microg/L perchlorate in the public drinking water. We tested the hypothesis that long-term exposure to perchlorate at these levels may cause a situation analogous to iodine deficiency, thus causing increases in thyrotropin (TSH) and thyroglobulin (Tg) levels and decreased levels of free thyroxine (FT4), in either the mother during the early stages of gestation or the neonate at birth, or in the fetus cause growth retardation. We found no increases in Tg or TSH and no decreases in FT4 among either the women during early pregnancy (16.1 +/- 4.1 weeks), late pregnancy (32.4 +/- 3.0 weeks), or the neonates at birth related to perchlorate in drinking water. Neonatal birth weight, length, and head circumference were not different among the three cities and were consistent with current U.S. norms. Therefore, perchlorate in drinking water at 114 microg/L did not cause changes in neonatal thyroid function or fetal growth retardation. Median urinary iodine among the entire cohort was 269 microg/L, intermediate between that of pregnant women in the United States at National Health and Nutrition Examination Survey (NHANES) I and at NHANES III and consistent with current World Health Organization (WHO) recommendations. Median breast milk iodine was not decreased in the cities with detectable perchlorate. Analysis of maternal urinary perchlorate excretion indicates an additional dietary source of perchlorate.

Perchlorate is known to suppress thyroid function by inhibiting uptake of iodide by the human thyroid at doses of 200 mg/day or greater. A study was conducted to investigate the potential effects of perchlorate in drinking water on thyroid function in newborns and school-age children. A total of 162 school-age children and 9784 newborns were studied in three proximate cities in northern Chile that have different concentrations of perchlorate in drinking water: Taltal (100 to 120 micrograms/L), Chañaral (5 to 7 micrograms/L), and Antofagasta (non-detectable: < 4 micrograms/L). Among schoolchildren, no difference was found in thyroid-stimulating hormone levels or goiter prevalence among lifelong residents of Taltal or Chañaral compared with those of Antofagasta, after adjusting for age, sex, and urinary iodine. No presumptive cases of congenital hypothyroidism were detected in Taltal or Chañaral; seven cases were detected in Antofagasta. Neonatal thyroid-stimulating hormone levels were significantly lower in Taltal compared with Antofagasta; this is opposite to the known pharmacological effect of perchlorate, and the magnitude of difference did not seem to be clinically significant. These findings do not support the hypothesis that perchlorate in drinking water at concentrations as high as 100 to 120 micrograms/L suppresses thyroid function in newborns or school-age children.