Thyroid antagonists and thyroid indicators in U.S. pregnant women in the Vanguard Study of the National Children's Study

Abstract

The sodium iodide-symporter (NIS) mediates uptake of iodide into thyroid follicular cells. This key step in thyroid hormone synthesis is inhibited by perchlorate, thiocyanate (SCN) and nitrate (NO3) anions. When these exposures occur during pregnancy the resulting decreases in thyroid hormones may adversely affect neurodevelopment of the human fetus. Our objectives were to describe and examine the relationship of these anions to the serum thyroid indicators, thyroid stimulating hormone (TSH) and free thyroxine (FT4), in third trimester women from the initial Vanguard Study of the National Children’s Study (NCS); and to compare urine perchlorate results with those in pregnant women from the National Health and Nutritional Examination Survey (NHANES). Urinary perchlorate, SCN, NO3, and iodine, serum TSH, FT4, and cotinine were measured and a food frequency questionnaire (FFQ) was administered to pregnant women enrolled in the initial Vanguard Study. We used multiple regression models of FT4 and TSH that included perchlorate equivalent concentration (PEC, which estimates combined inhibitory effects of the anions perchlorate, SCN, and NO3 on the NIS). We used multiple regression to model predictors of each urinary anion, using FFQ results, drinking water source, season of year, smoking status, and demographic characteristics. Descriptive statistics were calculated for pregnant women in NHANES 2001–2012. The geometric mean (GM) for urinary perchlorate was 4.04 μg/L, for TSH 1.46 mIU/L, and the arithmetic mean for FT4 1.11 ng/dL in 359 NCS women. In 330 women with completed FFQs, consumption of leafy greens, winter season, and Hispanic ethnicity were significant predictors of higher urinary perchlorate, which differed significantly by study site and primary drinking water source, and bottled water was associated with higher urinary perchlorate compared to filtered tap water. Leafy greens consumption was associated with higher urinary NO3 and higher urinary SCN. There was no association between urinary perchlorate or PEC and TSH or FT4, even for women with urinary iodine < 100 μg/L. GM urinary perchlorate concentrations in the full sample (n=494) of third trimester NCS women (4.03 μg/L) were similar to pregnant women in NHANES (3.58 μg/L).

Effect of perchlorate and thiocyanate exposure on thyroid function of pregnant women from South-West England: a cohort study

“…Previous studies examining association between perchlorate exposure and maternal thyroid function in pregnancy have shown inconsistent results (Table 4 ). Although some studies [ 6 – 8 , 12 ] found no association between UPC and maternal thyroid function, others have shown that UPC is associated with low maternal FT4 and/or high TSH [ 9 – 11 ]. Possible reasons for the discrepant results include diverse study populations, different iodine status, and variable gestational age at the time of perchlorate and thyroid function assessment.…”

“…This could lead to health hazards that may be more pronounced in vulnerable populations, such as pregnant women. Several studies have examined the effect of perchlorate and thiocyanate exposure during pregnancy on maternal thyroid function, however these showed conflicting results, and the potential adverse impact of exposure remains controversial [ 6 – 12 ].…”

Perchlorate – properties, toxicity and human health effects: an updated review

“…It seems, therefore, that the results obtained in this study cannot be unambiguously extrapolated to estimate the effect of perchlorate on the level of thyroid hormones in breastfed babies. Mortensen et al [114] investigated the effect of perchlorate, thiocyanate and nitrate on the plasma thyroid hormone concentration in women in the third trimester of pregnancy. The study group consisted of 359 women from various locations in the United States.…”

Development of Databases on Iodine in Foods and Dietary Supplements

“…Tobacco smoke presents another important goitrogenic exposure due to its thiocyanate content. As individuals are unlikely to be aware of ingestion of most environmental goitrogens, it may be useful to consider estimating exposure through the use of biomarkers [ 131 , 132 ].…”