2023: Fluoride exposure and hypothyroidism in a Canadian pregnancy cohort

All adverse health effects of fluoride are related to thyroid hormone metabolism.
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2023: Fluoride exposure and hypothyroidism in a Canadian pregnancy cohort

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Hall M, Lanphear B, Chevrier J, Hornung R, Green R, Goodman C, Ayotte P, Esperanza Angeles Martinez-Mier EA, Zoeller RT, Christine Till - "Fluoride exposure and hypothyroidism in a Canadian pregnancy cohort" Science of The Total Environment, Available online 9 February 2023, 161149
https://www.sciencedirect.com/science/a ... 9722082523
See PFPC Commentary on Goodman et al., 2022; same cohort (MIREC), and same criticism applies. 88% of the women took prenatal multivitamins which, in Canada, usually contain 220 μg of potassium iodide (= 170 I). As a result, many women have a much higher than "adequate" iodine intake which is reflected in the higher rates of subclinical hypothyroidism and auto-immune thyroid disease. [The daily iodine mean iodine intake was 444 μg, almost twice the recommended daily intake for pregnant women (250 μg/day). Mean UIC/Cr for the cohort was 344 μg/g, WHO upper limit of "adequate" unadjusted UIC is 250 μg/L. Iodine-induced hyperthyroid women (potential fluoride benefits) were not considered. "Primary Hypothyroid" group - showing alleged fluoride adverse effects - was mainly comprised of euthyroid women who were on thyroid hormone medication (T4) for pre-diagnosed hypothyroidism [72.9 %] - and had recently begun taking iodine-containing supplements. Many problems with this study, as in previous MIREC studies.

Please also see annotated reference list of studies cited in our Member York/MIREC FAQ: viewtopic.php?f=97&t=5022


Abstract

Background

While fluoride can have thyroid-disrupting effects, associations between low-level fluoride exposure and thyroid conditions remain unclear, especially during pregnancy when insufficient thyroid hormones can adversely impact offspring development.

Objectives

We evaluated associations between fluoride exposure and hypothyroidism in a Canadian pregnancy cohort.

Methods

We measured fluoride concentrations in drinking water and three dilution-corrected urine samples and estimated fluoride intake based on self-reported beverage consumption. We classified women enrolled in the Maternal-Infant Research on Environmental Chemicals Study as euthyroid (n = 1301), subclinical hypothyroid (n = 100) or primary hypothyroid (n = 107) based on their thyroid hormone levels in trimester one. We used multinomial logistic regression to estimate the association between fluoride exposure and classification of either subclinical or primary hypothyroidism and considered maternal thyroid peroxidase antibody (TPOAb) status, a marker of autoimmune hypothyroidism, as an effect modifier. In a subsample of 466 mother-child pairs, we used linear regression to explore the association between maternal hypothyroidism and child Full-Scale IQ (FSIQ) at ages 3-to-4 years and tested for effect modification by child sex.

Results

A 0.5 mg/L increase in drinking water fluoride concentration was associated with a 1.65 (95 % confidence interval [CI]: 1.04, 2.60) increased odds of primary hypothyroidism. In contrast, we did not find a significant association between urinary fluoride (adjusted odds ratio [aOR]: 1.00; 95%CI: 0.73, 1.39) or fluoride intake (aOR: 1.25; 95%CI: 0.99, 1.57) and hypothyroidism. Among women with normal TPOAb levels, the risk of primary hypothyroidism increased with both increasing water fluoride and fluoride intake (aOR water fluoride concentration: 2.85; 95%CI: 1.25, 6.50; aOR fluoride intake: 1.75; 95%CI: 1.27, 2.41). Children born to women with primary hypothyroidism had lower FSIQ scores compared to children of euthyroid women, especially among boys (B coefficient: −8.42; 95 % CI: −15.33, −1.50).

Discussion

Fluoride in drinking water was associated with increased risk of hypothyroidism in pregnant women. Thyroid disruption may contribute to developmental neurotoxicity of fluoride.
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