Fluoride & iodine

What we are reading

Gathered for the investigation and not yet written up. Each note says what the paper does and does not show.

1. Waugh DT. Fluoride Exposure Induces Inhibition of Sodium/Iodide Symporter (NIS) Contributing to Impaired Iodine Absorption and Iodine Deficiency.Int J Environ Res Public Health · 2019 · 16(6):108610.3390/ijerph16061086 · PMID 30917615

   The fullest statement of the displacement hypothesis, and the source most of the internet is downstream of. Proposed route: fluoride does not swap for iodine atom-for-atom, it suppresses the SODIUM-IODIDE SYMPORTER — the pump that drags iodide into the thyroid — partly by driving up TNF-alpha, IL-6, IL-1-beta, IFN-gamma and TGF-beta-1, all of which independently downregulate NIS, and partly via prolactin and megalin. IMPORTANT FOR HONESTY: this is a single-author narrative review by an environmental consultant, not primary data, and it hypothesises the mechanism rather than measuring it. It is the right place to get the hypothesis and the wrong place to stop.

2. Mudd AM, Ovando BJ, Daston GP. Probing the Biological Plausibility of Fluoride as an Endocrine Disruptor.Birth Defects Res · 2026 · 118(5):e7004610.1002/bdr2.70046 · PMID 42138003

   The direct experimental test of the displacement hypothesis, and it came back NEGATIVE. Sodium fluoride up to 300 micromolar produced NO change in iodide uptake through the sodium-iodide symporter; up to 10 micromolar it did not bind thyroid hormone receptors, did not inhibit thyroid peroxidase, and did not touch oestrogen, androgen, progesterone, PPAR, AhR, CAR, PXR, RAR or glucocorticoid receptors; up to 316 micromolar it did not alter oestrogen or testosterone synthesis. TWO THINGS MUST BE SAID TOGETHER. First: all three authors work for Procter & Gamble, which sells fluoride toothpaste, and that belongs on air in the same breath as the result. Second: the result is still the best direct measurement anyone has of this mechanism, and an in-vitro assay at fixed concentrations is not the same experiment as thirty years of drinking the stuff. Report the finding, report the funder, and do not pretend either one cancels the other.

3. Patial B, Thakur R. Exploring the Impacts of Fluoride-Induced Thyroid Toxicity: A Comprehensive Review.Biol Trace Elem Res · 2025 · 204(4):2594–261910.1007/s12011-025-04840-6 · PMID 41073679

   Reviews everything published on fluoride and the thyroid from 1976 to 2025. Reported effects: interference with iodine metabolism, lower T3 and T4, higher TSH, oxidative stress, downregulated antioxidant and thyroid genes, and histology showing follicular-cell apoptosis, necrosis and hyperplasia. Their framing — 'the thyroid gland in particular is highly susceptible to fluoride accumulation' — is worth quoting, but note that the weight of what they review is ANIMAL work at exposures well above 0.7 mg/L.

4. Peckham S, Lowery D, Spencer S. Are fluoride levels in drinking water associated with hypothyroidism prevalence in England? A large observational study of GP practice data and fluoride levels in drinking water.J Epidemiol Community Health · 2015 · 69(7):619–62410.1136/jech-2014-204971 · PMID 25714098

   The population study everyone cites: GP practices in the fully fluoridated West Midlands were nearly twice as likely to report high hypothyroidism prevalence as practices in unfluoridated Greater Manchester. It is ECOLOGICAL — practice-level, not person-level — and adjusted only for deprivation, so it cannot separate fluoride from everything else that differs between Birmingham and Manchester. Always pair it with the reply below; running it alone would be exactly the kind of one-sided citation this show exists to refuse.

5. Newton JN, Young N, Verne J, Morris J. Water fluoridation and hypothyroidism: results of this study need much more cautious interpretation.J Epidemiol Community Health · 2015 · 69(7):617–61810.1136/jech-2015-205917 · PMID 26068199

   Public Health England's response, published in the same issue. The core objection is a good one: iodine status, the actual driver of hypothyroidism, was not measured at all, and the two comparison regions differ in far more than their water. Worth reading alongside the 2017 follow-up exchange (PMID 28093451) in which neither side moves.

6. Griebel-Thompson AK, Sands S, Chollet-Hinton L, Christifano D, Sullivan DK, Hull H, Carlson SE. A Scoping Review of Iodine and Fluoride in Pregnancy in Relation to Maternal Thyroid Function and Offspring Neurodevelopment.Adv Nutr · 2023 · 14(2):317–33810.1016/j.advnut.2023.01.003 · PMID 36796438

   The most useful single sentence in this whole literature. These authors went looking for studies that measured BOTH iodine and fluoride in pregnancy — the only design that could actually test whether fluoride matters more in iodine-deficient women — and found exactly ONE. After seventy years of fluoridation and thirty years of iodine-deficiency research, one study has asked the obvious question. Their conclusion is four words long: more studies are needed.