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Fluoride exposure and thyroid hormone levels in pregnancy: The MIREC cohort.Abstract
Highlights
- Fluoride may disrupt thyroid function in pregnant women.
- Fluoride exposure was associated with alterations in maternal thyroid hormone levels.
- Urinary fluoride was associated with significantly higher TSH among those pregnant with females.
- Adjustment for maternal iodine status did not change the results.
Background
Fluoride exposure may increase the risk of hypothyroidism, but results from previous studies are inconsistent at low-level fluoride exposure (i.e., ? 0.7 mg/L). Human studies of fluoride and thyroid hormone levels in pregnancy are scarce.
Objectives
We examined associations between fluoride exposure and maternal thyroid hormone levels in a Canadian pregnancy cohort, with consideration for fetal sex-specific effects.
Methods
We measured fluoride concentrations in drinking water and spot urine samples collected during each trimester from 1876 pregnant women enrolled in the Maternal-Infant Research on Environmental Chemicals (MIREC) study. We also measured maternal thyroid stimulating hormone (TSH), free thyroxine (FT4), and total thyroxine (TT4) levels during the first trimester of pregnancy. We used linear and non-linear regression models to estimate associations between fluoride exposure and levels of TSH, FT4, and TT4. We explored effect modification by fetal sex and considered maternal iodine status as a potential confounder.
Results
A 1 mg/L increase in urinary fluoride was associated with a 0.30 (95 %CI: 0.08, 0.51) logarithmic unit (i.e., 35.0 %) increase in TSH among women pregnant with females, but not males (B = 0.02; 95 %CI: ?0.16, 0.19). Relative to women with urinary fluoride concentrations in the first quartile (0.05–0.32 mg/L), those with levels in the third quartile (0.49–0.75 mg/L) had higher FT4 and TT4 (i.e., inverted J-shaped associations), but the association was not statistically significant after adjustment for covariates (p = 0.06). Water fluoride concentration showed a U-shaped association with maternal FT4, whereby women with water fluoride concentrations in the second (0.13–0.52 mg/L) and third (0.52–0.62 mg/L) quartiles had significantly lower FT4 compared to those with levels in the first quartile (0.04–0.13 mg/L). Adjustment for maternal iodine status did not change the results.
Discussion
Fluoride exposure was associated with alterations in maternal thyroid hormone levels, the magnitude of which appeared to vary by fetal sex. Given the importance of maternal thyroid hormones for fetal neurodevelopment, replication of findings is warranted.
Graphical abstract
2.1. Participants
Pregnant women were enrolled in the Maternal-Infant Research on Environmental Chemicals (MIREC) Study (Arbuckle, 2013) between 2008 and 2011 from ten cities across Canada, seven of which add fluoride to drinking water (Toronto, Hamilton, Ottawa, Sudbury, Halifax, Edmonton, Winnipeg) and three of which do not (Vancouver, Montreal, Kingston). Women were eligible to participate if they were ? 18 years of age, able to communicate in English or French, and < 14 weeks’ gestation. Participants were considered ineligible if they had known fetal abnormalities, medical complications, or reported drug use. Of 2001 women recruited, 1983 consented to participate. Of these, 1885 (95.1 %) provided plasma samples in trimester one. We excluded those missing data on fetal sex (n = 9), for a final study sample of 1876 pregnant women.
The current study received approval from the research ethics boards at Health Canada and York University. All participants provided written informed consent at time of enrollment in MIREC.
4.2. Conclusions
In this Canadian pregnancy cohort, higher levels of fluoride exposure were associated with alterations in maternal thyroid hormone levels, the magnitude of which varied by fetal sex. Our findings make an important contribution to the growing body of evidence suggesting that higher levels of fluoride exposure in pregnancy may have adverse effects on maternal thyroid function. The implications of this work are of public health significance when considering the vital role of the maternal thyroid in supporting optimal fetal growth and neurodevelopment. Future studies in this area are warranted to replicate the current findings.
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Full-text study online at https://www.sciencedirect.com/science/article/pii/S016041202400028X
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