Fluoride and thyroid function: What is a safe level of fluoride in drinking water to protect the most vulnerable?

A recent systematic review and dose response meta-analysis, by Iamandii et al., considered the question "Does fluoride exposure affect thyroid function?" The authors concluded that, overall, exposure to high-fluoride drinking water appears to non-linearly affect thyroid function and increase TSH release in children, starting above a threshold of exposure, and to increase the risk of some thyroid diseases. The threshold of exposure for the TSH increase was 2 mg/L, or 2.5 mg/L when the studies with the best quality were considered. Although the lowest-observed-adverse-effect level (LOAEL) observed for TSH was a water fluoride level of approximately 2.5 mg/L, the changes in Total T3 and Total T4, a negative association, were present from very low levels, e.g. a water fluoride level of 0.2 mg/L and did not show a threshold effect. Fluoride in drinking water has been associated, by Hall et al., with an increased risk of hypothyroidism in pregnant women. 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. 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). The threshold, or point of departure, for fluoride in drinking water varies for different adverse effects such as the development of crippling skeletal fluorosis, dental fluorosis, an elevated TSH, a decreased Total T3 and Total T4, and impaired IQ in children. A recent paper by Grandjean et al. provides evidence that foetal toxicity can occur with a reduction in IQ when the maternal urinary fluoride is above 0.3 mg/L. A BMCL fluoride concentration in maternal urine during pregnancy of about 0.3 mg/L can be estimated to occur with a drinking water fluoride level of 0.24 mg/L, or approximately 0.3 mg/L. These findings suggest that, in order to protect the most vulnerable, a foetus in utero, public health attention should be given to elevated fluoride intakes during pregnancy, whether from drinking water, black tea, or other sources. [ABSTRACT FROM AUTHOR]

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