Context: Thyroid hormone is critical for fetal neurodevelopment. Perchlorate and thiocyanate decrease thyroidal iodine uptake by competitively inhibiting the sodium/iodide symporter. It is clear that perchlorate and thiocyanate anions can influence thyroid function. However, as pollutants in the environment, their impact is conflicting. Objective: The objective was to determine the effects of environmental perchlorate and/or thiocyanate exposure on thyroid function in first-trimester pregnant women. Design and Patients: A cross-sectional study was conducted in 200 pregnant Thai women with a gestational age of 14 weeks or less. Measures: Urinary iodide, perchlorate, thiocyanate, and serum thyroid function tests were measured. Results: The women were aged 28.6 ± 6.1 years and the mean gestational age was 9.6 ± 2.7 weeks. Median urinary iodide, perchlorate, and thiocyanate concentrations were 153.5 μg/L, 1.9 μg/L, and 510.5 μg/L, respectively. Using Spearman's rank correlation analyses, there were positive correlations between serum TSH and urine perchlorate to creatinine (r = 0.20, P = .005) and TSH and thiocyanate to creatinine ratios (r = 0.22, P = .001). There were negative correlations between free T4 and the perchlorate to creatinine ratio (r = -0.18, P = .01) and free T4 and the thiocyanate to creatinine ratio (r = -0.19, P = .008). In multivariate analyses adjusting for log thiocyanate to creatinine ratio, log iodide to creatinine ratio, and gestational age, log perchlorate to creatinine ratio was positively associated with log TSH (P = .002) and inversely associated with log free T4 (P = .002). Log thiocyanate to creatinine ratio was a significant positive predictor of log TSH (P = .02) in women with a urine iodide level of less than 100 μg/L. Conclusions: Low-level environmental exposure to perchlorate and thiocyanate is common in Thailand. Low-level exposure to perchlorate is positively associated with TSH and negatively associated with free T4 in first-trimester pregnantwomenusing multivariate analyses. In multivariate analyses, thiocyanate exposure is also positively associated with TSH in a subgroup of pregnant women with low iodine excretion. Copyright © 2014 by the Endocrine Society.