Lanphear's Substack - November 26, 2025 - "The Science Behind Fluoride and Why Exposure Matters"

[wendy]

November 26, 2025

On November 25, Prof. Bruce Lanphear, co-author of all MIREC fluoride studies to date, published an article on the newly released study by John Robert Warren and colleagues that examines how fluoride exposure in childhood drinking water relates to cognitive performance measured in adolescence and again decades later in older adulthood. Lanphear's central point was that fluoride exposure was assessed too crudely to support a strong conclusion.

• Warren JR, Rumore G, Kim S, Grodsky E, Muller C, Manly JJ, Brickman AM - "Childhood fluoride exposure and cognition across the life course" Science Advances 11(47):eadz0757 (2025)
  https://doi.org/10.1126/sciadv.adz0757

• Lanphear's article, titled "The Quiet Chemistry of Childhood - The Science Behind Fluoride and Why Exposure Matters", is available here:
  https://substack.com/home/post/p-179773925

When we attempted to post a comment, a notice indicated that only paid subscribers could comment directly on the article. We therefore posted our comments in response to one of Prof. Lanphear’s Substack Notes associated with the article.

The comments are reproduced below, and any published reply from Prof. Lanphear will be added here as well.

November 26, 2025 - Comment - PFPC
https://substack.com/@pfpc/note/c-181603433

Interesting article. Besides the possibility that the Warren study might be severely lacking, numerous statements made here are perplexing and perhaps you could address them.

1. Water fluoride

In your article, you criticize the Warren study because it used water fluoride concentrations as an exposure measure, which you deem “simply too crude.”

Yet in your own studies (Hall et al., 2023; 2024) you repeatedly insist that municipal water fluoride is “relatively constant” and “more likely to be indicative of chronic fluoride exposure, and perhaps body burden, than urinary fluoride,” and you rely on water fluoride for your hypothyroidism analyses. Could you explain why you present water fluoride as a relatively constant and superior marker of chronic exposure when it supports fluoride-thyroid associations in your own MIREC papers, yet downplay essentially the same water based exposure metric in the Warren study when it does not show fluoride-related cognitive harm? IQ can change throughout life, and in high iodine areas it has been found to decrease with age, so adolescent and adult exposures are certainly not irrelevant exposure windows either.

2. Green study

I am also curious why you refer to Green et al. - a study you co-authored - as “their findings” and not “our findings.” You write: “Their findings are striking: a 3.7-point decrease in IQ for every 1 mg/L increase in fluoride intake and, among boys, a 4.5-point decrease in IQ per 1 mg/L increase in maternal urinary fluoride.”

As has been discussed numerous times, your 2019 study with Green did not adjust for iodine intake, a critical confounder and modifier in fluoride toxicity. It is now established that close to 90% of the MIREC women took a prenatal supplement containing on average about 220 µg of iodine. Your own data showed that urinary iodine was clearly increased in fluoridated areas, as were maternal thyroid antibodies (Hall et al., 2024). A rise in maternal antibodies is a classic sign of overexposure to iodine.
viewtopic.php?p=8250#p8250

In subsequent efforts by your team to address the fluoride-iodine issue (Goodman et al., 2022), a highly questionable self-devised reference range for “adequate” iodine intake was used - a UIC/Cr range of 200–600 µg/g - even though the WHO reference range for adequate iodine intake in pregnancy is 150–249 µg/L UIC. U-shaped effects of iodine are well established. A UIC above 250 µg/L is associated with an increase in hypothyroidism and a rise in autoimmune thyroid disease, which in turn is associated with loss of IQ in the offspring (e.g. Derakhshan et al., 2018; Wasserman et al., 2012). Sex-specific differences in the effects of maternal thyroid dysfunction on offspring outcomes have long been reported.

3. NTP review

The NTP review, repeatedly referred to as a “landmark analysis,” also did not address the iodine issue anywhere near correctly or adequately. It even rewrote the study protocol after NASEM criticism in order to avoid properly addressing this matter. This was discussed extensively in our letter to the NTP in 2023.
https://substack.com/@pfpc/p-123925262

The NTP review (Taylor et al., 2025) falsely claimed to have excluded studies where “iodine intake was differential,” but this is demonstrably incorrect.

The vast majority of the so-called “high quality” studies used by the NTP were conducted under conditions of high iodine intake. Since our letter, numerous studies have been published confirming the high iodine intake in several “high quality” studies from China used by the NTP in its assessment (for example Xia et al., 2025; Liu et al., 2024). As many Chinese studies in humans and animals have shown, iodine toxicity must be addressed first when confronted with low or “normal” fluoride intake, because in such settings high iodine is the predominant driver of thyroid dysfunction and related neurodevelopmental outcomes.

REFERENCES

Derakhshan A, Korevaar TIM, Taylor PN, Levie D, Guxens M, Jaddoe VWV, Nelson SM, Tiemeier H, Peeters RP - "The Association of Maternal Thyroid Autoimmunity During Pregnancy With Child IQ" J Clin Endocrinol Metab 103(10):3729-3736 (2018)
https://eprints.gla.ac.uk/165669/

Goodman CV, Hall M, Green R, Chevrier J, Ayotte P, Matinez-Mier EA, McGuckin T, Krzeczkowski J, Flora D, Hornung R, Lanphear B, Till C - "Iodine Status Modifies the Association between Fluoride Exposure in Pregnancy and Preschool Boys’ Intelligence" Nutrients 14(14):2920 (2022)
https://www.mdpi.com/2072-6643/14/14/2920

Hall M, Hornung R, Chevrier J, Ayotte P, Lanphear B, Till C - "Fluoride Exposure and Thyroid Hormone Levels in Pregnancy: The MIREC Cohort" Environment International 108442 (2024)
https://doi.org/10.1016/j.envint.2024.108442

Hall M, Lanphear B, Chevrier J, Hornung R, Green R, Goodman C, Ayotte P, Martinez-Mier EA, Zoeller RT, Till C - "Letter to the editor regarding Hall et al. (2023): Fluoride exposure and hypothyroidism in a Canadian pregnancy cohort" Science of the Total Environment 933:173121 (2024)
https://doi.org/10.1016/j.scitotenv.2024.173121

Liu S, Yu X, Xing Z, Ding P, Cui Y, Liu H - "The Impact of Exposure to Iodine and Fluorine in Drinking Water on Thyroid Health and Intelligence in School-Age Children: A Cross-Sectional Investigation" Nutrients 16(17):2913 (2024)
https://doi.org/10.3390/nu16172913

Wasserman EE, Pillion JP, Duggan A, Nelson K, Rohde C, Seaberg EC, Talor MV, Yolken RH, Rose NR - "Childhood IQ, hearing loss, and maternal thyroid autoimmunity in the Baltimore Collaborative Perinatal Project" Pediatr Res 72(5):525-30 (2012) doi: 10.1038/pr.2012.117
https://www.nature.com/articles/pr2012117

Xia Y, Ye Y, Liu M, Wang Y, Shang L, Wang P, Ding Z - "Effect of combined high iodine-fluorine water exposure on the occurrence of dental fluorosis in school-age children: a cross-sectional study from rural Jiangsu, China" Environ Geochem Health 47(9):369 (2025) doi: 10.1007/s10653-025-02685-5.
https://link.springer.com/article/10.10 ... 25-02685-5

[wendy]

November 26, 2025 - Response - Bruce Lanphear
https://substack.com/profile/17782996-b ... -181647617

PFPC Canada: Water fluoride can be a useful exposure measure, even though it doesn’t capture other fluoride sources such as tea, toothpaste, or supplements. The issue with Warren’s measure is not the concept of using water data—it’s that their exposure proxy was ecologic, based on whether a community was fluoridated decades earlier, with no evidence that the mothers or children actually drank that water during key developmental windows. That makes it a crude proxy, especially for a developmental neurotoxicant where timing is critical.

By contrast, in the MIREC Study we obtained measured fluoride concentrations directly from the municipal water treatment plants serving each participant’s home. And for our analyses of fluoride intake, we restricted the sample to women who reported drinking their tap water, which increased confidence that the measured fluoride reflected actual exposure.

To be clear, the question Warren posed—whether adolescents who grew up in fluoridated communities perform differently on cognitive tests—is a valid and interesting one. But their community-level exposure measure simply isn’t precise enough to dismiss the stronger evidence built from biomarker-based studies.

As I’ve mentioned before, in Green et al, “we” chose to test a specific hypothesis about the association of fluoride with IQ tests. We may or may not have had the urinary iodine measured by 2019. In either case, we treated it as a separate question or hypothesis.

I can’t speak to the question of iodine in the NTP. But the 2025 meta-analysis was an extraordinary effort over many years that underwent a gauntlet of reviews and criticisms, and it marks a turning point in our understanding of fluoride neurotoxicity.

Finally, I am overdue to set up a meeting with you and the MIREC team.

[wendy]

November 28, 2025 - Response - PFPC
https://substack.com/profile/46616808-p ... -182250698

I have trouble understanding your claim that the MIREC exposure assessment is more precise than Warren’s.

I provided the example of your own thyroid studies where you insist that municipal water fluoride is “relatively constant” and “more likely to be indicative of chronic fluoride exposure, and perhaps body burden, than urinary fluoride,” and where you relied on water fluoride as the main exposure metric for your hypothyroidism analyses.

In fact, your situation appears worse than Warren’s. At least Warren reconstructed childhood exposure using residential histories across time. In MIREC, about 75% of the women you classified as “primary hypothyroid” were NOT hypothyroid at the time the urine and water samples were collected. They were placed in that group because they had received a diagnosis of primary hypothyroidism at some earlier point, BEFORE the study took place. Nowhere is it disclosed when that diagnosis was made, where they were living at the time, what water they were consuming, or whether they were even in a fluoridated area. Yet you still treat a single municipal value at the CURRENT pregnancy address plus a tap water self-report as if it reflected the relevant long-term exposure for that diagnosis.

Your answer on Green/iodine is also not reassuring. Saying you “may or may not have had the urinary iodine measured by 2019” misses the point entirely.

Iodine and thyroid status are not a “separate question” when you claim to be testing a fluoride-IQ hypothesis in pregnancy. Genuine hypothesis testing must be grounded in current knowledge. It is already established that iodine/thyroid function is central to brain development and that both low and high iodine intake can alter IQ. In that context, a fluoride-IQ analysis that ignores iodine is a fundamental design flaw, especially given that current knowledge includes close to 800 studies showing fluoride effects on thyroid hormone metabolism across all levels of iodine status, including its pharmacological use to treat iodine-induced thyroid dysfunction. If urinary iodine was not available in 2019, the honest conclusion would have been that you could not meaningfully disentangle fluoride from iodine effects, rather than proceeding as if iodine did not matter. I also note that you did not address the concern about the highly inappropriate 200–600 µg/g UIC/Cr “adequate” range for pregnant women in your 2022 paper on fluoride-IQ that supposedly “adjusted” for iodine intake.

I find it very troubling that in your Substack articles you continue to cite your Green study as key evidence on fluoride and IQ, even though you know from your own data that the fluoridated areas also had higher iodine levels and higher maternal thyroid antibodies. That is not acceptable scientifically.

The NTP review remains THE most over-hyped, flawed, and incompetent analysis yet seen on this subject. We followed the NTP work from the beginning. Our first submission to the NTP was made in 2016, after the initial literature review was published, already showing the misguided approach which was later to manifest.

As stated many times before, the majority of its so-called “high quality” IQ studies were conducted in environments of high iodine intake - including your own MIREC studies where iodine intake from prenatal supplements was introduced to an existing stable background of fluoride intake. The NTP was explicitly informed of this and chose not to revisit its analyses - earlier it rewrote its own protocol to avoid dealing with this issue. The fact that a report went through a gauntlet of reviews - none of which addressed this matter - does not change the fact that this work is built on a mix of studies that either never properly characterized iodine exposure, or did so and were then misinterpreted, or high iodine findings were simply ignored. On that basis it cannot provide any reliable estimate of fluoride neurotoxicity.

You say you “can’t speak to the question of iodine in the NTP,” yet you promote the work as an “extraordinary effort” and a “turning point in our understanding of fluoride neurotoxicity.” You cannot endorse the work as a landmark while simultaneously disclaiming responsibility for the single most important confounder and modifier in the very studies it synthesizes. Either iodine is central, in which case the NTP’s handling of it must be addressed head-on, or it is not, in which case your entire fluoride-IQ framework collapses against what is known about thyroid and neurodevelopment.

To underline the absurdity of this, some of the very studies that the NTP relied on in its meta-analyses to assess fluoride neurotoxicity are used in Chinese meta-analyses as evidence for iodine-related neurotoxicity - not fluoride. That certainly should give anyone serious about causality some pause.

To top it off, one of these meta-analyses is then cited in your own work as evidence that excess iodine can reduce IQ. How can things be any more surreal and preposterous?

[wendy]

UPDATE: December 5, 2025

No further reply was received from Bruce Lanphear.