64 l © 2025 American Dental Association
Section 2: Safety
35. Does the ingestion of fluoridated water at recommended levels
adversely affect the thyroid gland or its function?
Answer
No. The best available scientific evidence indicates optimally fluoridated water does not have
an adverse effect on the thyroid gland or its function.
Fact
Several systematic reviews and individual studies completed in the last 15 years have looked at a
possible association between exposure to fluoride and thyroid function. Much of the literature that
reports an association between fluoride and a risk of thyroid disease discusses the risks of excessive
fluoride exposure33,189,300 at levels well above those currently recommended by the ADA and most
public health institutions, and thus should not be used as evidence against optimal water fluoridation.
The following reviews support the safety of community water fluoridation at recommended levels:
National Toxicology Program Monograph (2024)
The NTP Monograph301 systematically reviewed human, animal, and mechanistic studies on the
thyroid gland and hormones on the extent and quality of the evidence linking fluoride exposure to
neurodevelopmental and cognitive effects in humans (Question 41). Changes in thyroid hormones
have been proposed as a potential mechanism for neurodevelopmental effects,39,302 which is why
thyroid effects were also evaluated in the final report. The NTP report evaluated eight low risk-of-
bias studies and 16 high risk-of-bias studies. The studies examined thyroid hormones—the thyroid-
stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4)—as markers of thyroid gland
function. The evidence includes a mix of findings, with some variability across studies based on factors
such as fluoride concentration, age, and study design. When examining associations between fluoride
exposure and thyroid hormones (TSH, T3, and T4), studies that analyzed changes across all three
hormones reported varied results, including increases, decreases, or no changes in hormone levels.
These studies also highlighted age-related differences in the associations between fluoride exposure
and thyroid hormones. The findings indicate that while high levels of fluoride exposure may influence
thyroid hormones, the evidence does not conclusively demonstrate that consuming fluoridated water
at recommended levels adversely affects the thyroid gland or its function. The variability in study
results and the complex interplay between the thyroid and other physiological systems suggest that
any potential effects are not straightforward. Therefore, based on current evidence, fluoridated water
at recommended concentrations is not clearly linked to negative impacts on thyroid health.301
Iamandii et al. (2023)
A 2023 systematic review300 of fluoride exposure and thyroid function evaluated data from studies
conducted in Asia, Europe, Africa, and Canada. The authors examined low and high levels of naturally
fluoridated water reported in the original papers. Thyroid function, as measured by TSH and T4,
showed little or no impact related to water fluoride. It is important to note that community water
fluoridation was not evaluated, and no US studies were included. Furthermore, results showed that
levels of natural fluoride at the same level as that recommended in the United States (0.7 ppm)
showed no negative impact on thyroid function or thyroid disease.300
Section 2: Safety
35. Does the ingestion of fluoridated water at recommended levels
adversely affect the thyroid gland or its function?
Answer
No. The best available scientific evidence indicates optimally fluoridated water does not have
an adverse effect on the thyroid gland or its function.
Fact
Several systematic reviews and individual studies completed in the last 15 years have looked at a
possible association between exposure to fluoride and thyroid function. Much of the literature that
reports an association between fluoride and a risk of thyroid disease discusses the risks of excessive
fluoride exposure33,189,300 at levels well above those currently recommended by the ADA and most
public health institutions, and thus should not be used as evidence against optimal water fluoridation.
The following reviews support the safety of community water fluoridation at recommended levels:
National Toxicology Program Monograph (2024)
The NTP Monograph301 systematically reviewed human, animal, and mechanistic studies on the
thyroid gland and hormones on the extent and quality of the evidence linking fluoride exposure to
neurodevelopmental and cognitive effects in humans (Question 41). Changes in thyroid hormones
have been proposed as a potential mechanism for neurodevelopmental effects,39,302 which is why
thyroid effects were also evaluated in the final report. The NTP report evaluated eight low risk-of-
bias studies and 16 high risk-of-bias studies. The studies examined thyroid hormones—the thyroid-
stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4)—as markers of thyroid gland
function. The evidence includes a mix of findings, with some variability across studies based on factors
such as fluoride concentration, age, and study design. When examining associations between fluoride
exposure and thyroid hormones (TSH, T3, and T4), studies that analyzed changes across all three
hormones reported varied results, including increases, decreases, or no changes in hormone levels.
These studies also highlighted age-related differences in the associations between fluoride exposure
and thyroid hormones. The findings indicate that while high levels of fluoride exposure may influence
thyroid hormones, the evidence does not conclusively demonstrate that consuming fluoridated water
at recommended levels adversely affects the thyroid gland or its function. The variability in study
results and the complex interplay between the thyroid and other physiological systems suggest that
any potential effects are not straightforward. Therefore, based on current evidence, fluoridated water
at recommended concentrations is not clearly linked to negative impacts on thyroid health.301
Iamandii et al. (2023)
A 2023 systematic review300 of fluoride exposure and thyroid function evaluated data from studies
conducted in Asia, Europe, Africa, and Canada. The authors examined low and high levels of naturally
fluoridated water reported in the original papers. Thyroid function, as measured by TSH and T4,
showed little or no impact related to water fluoride. It is important to note that community water
fluoridation was not evaluated, and no US studies were included. Furthermore, results showed that
levels of natural fluoride at the same level as that recommended in the United States (0.7 ppm)
showed no negative impact on thyroid function or thyroid disease.300
