Association between prenatal fluoride exposure and thyroid hormone levels in INMA-Gipuzkoa cohort

Background

Artificial fluoridation of community drinking water is widely used as Public Health strategy to improve community dental health, being the main source of chronic fluoride exposure. However, it has been associated with adverse effects, including potential thyroid dysfunction. Thyroid hormones are essential during pregnancy for maternal health and fetal development, and even mild dysfunction has been associated with adverse outcomes. However, epidemiological evidence is still limited and inconsistent. The aim of this work is to assess the association of prenatal fluoride exposure in maternal and newborn thyroid hormone levels.

Methods

This study is part of the INMA Project, in which 553 pregnant women and 378 mother–child pairs were included. Prenatal fluoride exposure was assessed using urinary fluoride at weeks 12 and 32. Maternal thyroid hormones (TSH, FT4, TT3) were measured in the first trimester, and neonatal TT4 was obtained at 48 hours after birth. Models were adjusted for relevant covariates (DAG) and iodine status using urinary iodine and serum thyroglobulin (Tg). Stratified and interaction analyses were conducted.

Results

In mothers, no associations were observed between fluoride exposure and maternal hormones in iodine-adjusted models or in analyses stratified by iodine status (sufficient/insufficient), and no statistically significant interactions were found. Only the model among women with Tg levels below the median showed an inverse association between fluoride and maternal TT3 (β = −0.122; p = 0.031). In newborns, no model showed a significant association between fluoride exposure and thyroid hormones.

Conclusions

Prenatal fluoride exposure was not consistently associated with thyroid hormone. The observed association in women with lower Tg suggests potential susceptibility related to iodine status. Further research is needed, incorporating robust iodine biomarkers such as Tg, to clarify the role of fluoride in thyroid function during pregnancy and birth.

Key messages

• Pregnancy and birth are highly vulnerable periods for thyroid function, and environmental exposures such as fluoride may have potential impacts on hormonal regulation.

• Further studies are needed to clarify and strengthen the role of thyroglobulin as a sensitive biomarker of iodine status, supporting its use in epidemiological and public health research.