Fluoride Exposure Linked to Altered Thyroid Shape and Hormones in Children

Fluoride has long been celebrated as a cornerstone of public health dentistry, but a new study from China adds a surprising twist to the debate over how much exposure children should receive. Researchers at the Tianjin Centers for Disease Control and Prevention report that in a region where iodine nutrition is adequate, higher fluoride levels in children’s bodies are associated with measurable changes in both the size of the thyroid gland and the circulating levels of thyroid hormones. The findings, published in the journal Environmental Health, challenge the assumption that adequate iodine intake fully shields the developing thyroid from fluoride’s influence.

The cross-sectional study included 751 children aged 8 to 10 years living in iodine-adequate areas of Tianjin, a major coastal municipality in northern China. To capture fluoride exposure as comprehensively as possible, the team relied on two complementary biomarkers. Urinary fluoride concentration served as an indicator of recent total intake from all sources, including drinking water, food, and dental products. Dental fluorosis, the characteristic mottling of tooth enamel caused by excessive fluoride during tooth development, provided a record of cumulative exposure accumulated over years. Urinary iodine was also measured to confirm that children had sufficient iodine nutrition, the single most important dietary factor for thyroid health.

Thyroid outcomes were assessed along two distinct dimensions. Ultrasound imaging determined thyroid volume, allowing the researchers to identify goiter, an enlargement of the gland that has historically been the hallmark of iodine deficiency. In parallel, blood tests quantified three key markers of thyroid function: free triiodothyronine (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH), each classified as low, normal, or elevated. This dual approach, combining morphology and function, gave the investigators a multidimensional picture of thyroid health that few previous fluoride studies have achieved.

Statistical models adjusted for 33 potential confounders revealed a pattern that defies simple expectations. Both dental fluorosis and urinary fluoride were associated with reduced thyroid volume, with dental fluorosis showing a particularly strong inverse relationship. Correspondingly, children with higher fluoride exposure had lower odds of goiter. At first glance, a smaller thyroid with less goiter might sound like good news, but the functional markers told a more complicated story. Higher urinary fluoride was linked to lower levels of FT4, a critical hormone governing metabolism throughout the body, and to reduced risk of elevated TSH, while simultaneously increasing the risk of abnormally low FT3 by 63 percent per unit increase in exposure.

Restricted cubic spline analyses, a flexible modeling technique capable of revealing nonlinear dose-response relationships, sharpened the picture further. Thyroid volume declined steadily as urinary fluoride rose, plateauing only at approximately 7.45 milligrams per liter. FT4, by contrast, dropped rapidly at lower exposure levels and leveled off at around 2.87 milligrams per liter, suggesting that the hormone axis is sensitive to fluoride even at comparatively modest intakes. For the categorical outcomes, the risks of goiter and elevated TSH decreased monotonically with rising urinary fluoride, crossing the line of no association at roughly 1.1 milligrams per liter, while the risk of low FT3 increased progressively and crossed that same threshold at approximately the same concentration. That convergence near 1.1 milligrams per liter may prove to be one of the most consequential numbers in the study, hinting at where thyroid effects begin to emerge in a population with adequate iodine.

Perhaps the most striking results came when the researchers combined the two exposure indicators. Children who showed both dental fluorosis and elevated urinary fluoride, meaning they carried evidence of both long-term and recent exposure, exhibited the most pronounced reductions in FT4 and goiter risk. This cumulative-and-recent exposure framework suggests that fluoride’s impact on the thyroid is not a snapshot phenomenon but the product of sustained biological pressure across sensitive developmental windows, including the period of tooth formation when the child’s endocrine system is still maturing.

Sensitivity analyses strengthened confidence in the central findings. When the researchers excluded children with extreme iodine nutrition from the dataset, the associations for thyroid volume and FT4 remained robust, indicating that the observed effects were not artifacts of iodine status. This is a crucial point because iodine deficiency is the classic driver of goiter and thyroid dysfunction, and any fluoride study conducted in an iodine-poor region would struggle to disentangle the two exposures. By focusing specifically on iodine-adequate areas of Tianjin, the study isolates fluoride as the exposure of interest with unusual clarity.

The biological mechanisms behind these associations remain an open question, but plausible pathways deserve attention. Fluoride is a highly electronegative halogen with known affinity for calcium-containing tissues, and experimental work has suggested it can generate reactive oxygen species, disrupt iodine uptake by the thyroid’s sodium-iodide symporter, and interfere with the conversion of thyroxine to the more active triiodothyronine. The pattern observed in this study, with FT4 falling while FT3 deficiency risk rises, is consistent with such a mechanism, though the cross-sectional design of the research cannot establish causation or confirm the underlying physiology directly.

The inverse relationship between fluoride and goiter also warrants careful interpretation. Goiter typically represents the thyroid’s compensatory response to insufficient hormone production, often driven by iodine scarcity. A fluoride-associated reduction in thyroid volume and goiter risk does not necessarily indicate healthier glands; it may reflect a different pattern of thyroid adaptation or suppression that manifests in hormone levels rather than gland size. The authors emphasize that evaluating fluoride health risks requires considering multiple thyroid indicators and complementary exposure biomarkers rather than relying on any single outcome measure.

As water fluoridation policies continue to be debated in many countries, this study adds a data-rich, iodine-controlled data point to a growing body of evidence linking fluoride to thyroid outcomes in children. It does not settle questions about safe exposure thresholds for populations, and the observational design means reverse causation and residual confounding cannot be entirely excluded. Nevertheless, the detailed dose-response characterization, the dual-biomarker exposure assessment, and the iodine-adequate study population make this one of the most informative investigations of fluoride and child thyroid health to date, and it is likely to inform both regulatory risk assessments and future longitudinal research on this enduring public health question.

Subject of Research: Association of fluoride exposure with thyroid morphology and function in children living in iodine-adequate areas of Tianjin, China

Article Title: Association of fluoride exposure with thyroid morphology and function in children from iodine-adequate areas: a cross-sectional study in Tianjin, China

Article References: Duan, Y., Wang, Y., Li, F., Li, W., Zhang, D., & Cui, Y. (2026). Association of fluoride exposure with thyroid morphology and function in children from iodine-adequate areas: a cross-sectional study in Tianjin, China. Environmental Health. https://doi.org/10.1186/s12940-026-01337-0

Image Credits: AI Generated

DOI: 10.1186/s12940-026-01337-0

Keywords: fluoride exposure, thyroid function, children's health, dental fluorosis, urinary fluoride, thyroid volume, FT4, FT3, TSH, iodine status, dose-response relationship, environmental health