A consistent body of animal and human research shows that fluoride exposure worsens the impact of an iodine deficiency. Iodine is the basic building block of the T3 and T4 hormones and thus an adequate iodine intake is essential for the proper functioning of the thyroid gland. When iodine intake is inadequate during infancy and early childhood, the child’s brain can suffer permanent damage, including mental retardation.

Fluoride, Low Iodine, & IQ — Human Studies

In China, researchers have repeatedly found that an iodine deficiency coupled with fluoride exposure produces a significantly more damaging effect on neurological development than iodine deficiency alone. In the first study to investigate the issue, Ren reported that:

“From the results it is evident that disrupted child intellectual development is among the effects on the human body from a harmful environment containing both high fluoride and low iodine, and this disruption is clearly much more serious than the effects of iodine deficiency alone.”
SOURCE: Ren D, et al. (1989). A study of the intellectual ability of 8-14 year-old children in high fluoride, low iodine areas. Chinese Journal of Control of Endemic Diseases 4(4):251 (republished in Fluoride 2008; 41:319-20).

Since Ren’s study, other research teams have reported the same result. In 1991, a UNICEF-funded study concluded that fluoride levels of just 0.9 ppm (less than the level added to many water supplies for fluoridation) were sufficient to worsen the effects of iodine deficiency. The authors found that, when compared to children with iodine deficiency in a low-fluoride area, the children with iodine deficiency in the 0.9 ppm area had increased TSH levels, reduced T3, reduced intelligence, retarded bone development, and reduced hearing. According to the authors:

“Statistically significant differences existed between these areas, suggesting that a low iodine intake coupled with high fluoride intake exacerbates the central nervous lesions and the somatic developmental disturbance of iodine deficiency.”
SOURCE: Lin Fa-Fu; et al (1991). The relationship of a low-iodine and high-fluoride environment to subclinical cretinism in Xinjiang. Endemic Disease Bulletin 6(2):62-67 (republished in Iodine Deficiency Disorder Newsletter Vol. 7(3):24-25).

In 1994, Xu and colleagues measured the IQ rates of children living in 8 areas with differing levels of both iodine and fluoride in exposure. Of all the areas studied, the region with the high fluoride/low iodine content had the lowest IQ. In addition, when compared against the low-iodine area, the high fluoride/low iodine area had a significantly higher rate of thyroid swelling. According to the authors:

“A higher chance of one being affected by thyroid swelling is likewise more prevalent in regions containing a high amount of fluoride but low amount of iodine, and regions where a relatively lower amount of iodine is detected. We believe that in a region where the level of iodine is low, but fluoride is significantly elevated, the level of toxicity in thyroid swelling could increase.”
Xu Y, et al. (1994). The effect of fluorine on the level of intelligence in children. Endemic Disease Bulletin 9(2):83-84.

Similarly, in 2001, Hong and colleagues reported:

“The IQ results of this study show no significant difference between the average IQs of those children from the high fluoride only areas and the high fluoride/high iodine areas, however the result from the high fluoride/low iodine group show statistically significant differences as compared to that of the low fluoride/low iodine group.”
SOURCE: Hong F, et al. (2001). Research on the effects of fluoride on child intellectual development under different environments. Chinese Primary Health Care 15(3):56-57 (republished in Fluoride 2008; 41(2):156–60).

Spurred by these findings, Chinese researchers have begun conducting animal studies to further understand the neurological effects of iodine deficiency when coupled with fluoride exposures. Under controlled laboratory conditions, the studies have confirmed that the combined effect of iodine deficiency and fluoride is worse than either by itself.

In 2004, Wang and colleagues reported that:

“In comparison with control rats, the learning and memory ability of the offspring rats was depressed by high fluoride, low iodine, or the combination of high fluoride and low iodine. Brain protein was decreased by low iodine and even more by the combined interaction of high fluoride and low iodine. The activity of cholinesterase (ChE) in the brain was affected to some extent by high fluoride and low iodine but was especially affected by high fluoride and low iodine together.”
SOURCE: Wang J, et al. (2004). Effects of high fluoride and low iodine on biochemical indexes of the brain and learning-memory of offspring rats. Fluoride 37(4): 201-208.

In another study of learning and memory, Hong and colleagues reported that mice with high fluoride/low iodine exposure committed significantly more errors in the study’s test experiment than mice with either low iodine, or high fluoride, exposure alone. The authors also found that the level of “acetylcholinesterase (TchE) activity in the group of high fluoride and low iodine decrased significantly” in the high fluoride/low iodine group. [See study]

Other animal studies reporting interactive effects of fluoride and iodine on the brain (and thyroid) include:

• Ge Y, et al. (2011). Proteomic analysis of brain proteins of rats exposed to high fluoride and low iodine. Archives of Toxicology 85(1):27-33.
• Ge Y, et al. (2005a). Comet assay of DNA damage in brain cells of adult rats exposed to high fluoride and low iodine. Fluoride 38(3):209-14.
• Ge Y, et al. (2005b). DNA damage in thyroid gland cells of rats exposed to long-term intake of high fluoride and low iodine. Fluoride 38(4): 318-323.
• Shen X, Zhang Z, Xu X. (2004). [Influence of combined iodine and fluoride on phospholipid and fatty acid composition in brain cells of rats] Wei Sheng Yan Jiu. 33(2):158-61.
• Wang J, Ge Y, Ning H, Wang S. (2004). Effects of high fluoride and low iodine on biochemical indexes of the brain and learning-memory of offspring rats. Fluoride 37(4): 201-208.

Fluoride, Low Iodine, and Thyroid Disease:

The interactive effects of fluoride and low iodine on neurological health is consistent with other research showing that fluoride intensifies the anti-thyroid effects of iodine deficiency, and vice versa.

In an animal study by Guan (1988), fluoride’s effect on thyroid function occurred at lower doses, and was more severe, when the animals (rats) had an iodine deficiency. Similarly, the effects of iodine deficiency were more severe with fluoride exposure, than without. According to the authors:

“This study reveals that the degree of impairment of thyroid morphology and function is related with the amount of fluorine taken by rats. Goiter occurs in rats with iodine deficiency. Damage to the thyroid is observed in rats on iodine deficient diet and highly fluorinated water [30 ppm]. These changes are much more severe than in rats on a normal level iodine diet and highly fluorinated water. This seems to suggest that competitive antagonistic action exists between fluorine and iodine in the thyroid gland.”
SOURCE: Guan ZZ, et al. (1988). Synergistic action of iodine-deficiency and fluorine-intoxication on rat thyroid. Chinese Medical Journal 101(9):679-84.

In addition to Guan’s study, an animal study by Zhao et al (1998) found that fluoride and low iodine have “mutually interacting effects” on the thyroid gland, as evident by changes in thyroid weight, time-specific alterations in thyroid hormone levels, increased bone fluoride content, and increased severity of dental fluorosis. As with other studies, Zhao found that fluoride has interactive effects with iodine excess as well. [See study]

More recently, a team of Russian researchers studied a population with iodine deficiency that was exposed to varying levels of fluoride air pollution. The team found that indices of thyroid disease, including stunted growth and thyroid swelling, were more severe, and prophylactic measures less effective, in the population with heavier exposure to fluoride pollution. According to the authors:

“Natural iodine deficiency and ambient air pollution with fluorine compounds were examined for their combined influence on the prevalence and severity of iodine-deficiency disorders. The excess intake of fluorine was shown to increase the incidence of thyroid diseases and to lower anthropometric indices in children. The preventive measures performed to eliminate iodine-deficiency disorders under intensive ambient air pollution with fluorine compounds were found to be insufficiently effective.”
SOURCE: Gas’kov AIu, et al. (2005). [The specific features of the development of iodine deficiencies in children living under environmental pollution with fluorine compounds]. [Article in Russian] Gig Sanit. 2005 Nov-Dec;(6):53-5.

Fluoride, Low Iodine, and Dental Fluorosis

As noted above, the animal study by Zhao (1998) found that iodine deficiency worsened the severity of dental fluorosis in the fluoride-treated rats. In other words, rats given the same level of fluoride had more severe fluorosis if they had a co-existing iodine deficiency. Consistent with this finding, the study by Xu (1994) found far higher rates of dental fluorosis in a population with low iodine exposure, than a similar population with adequate iodine exposure. Although both communities had 0.8 ppm fluoride in the water, the rate of dental fluorosis was 89% in the low-iodine area, which was more than double the fluorosis rate (40%) in the area with adequate iodine.

More recently, a research team in Mexico reported a high rate of fluorosis in an area known for iodine deficiency. (Pontigo-Loyola 2008). Since the rate of fluorosis was higher than would be expected under normal circumstances, the authors suggested that iodine deficiency could be one of the factors contributing to the high rate. According to the authors,

“The hypothesized relationship between iodine deficiency and increased prevalence of fluorosis appears to be relevant to Hidalgo.”
SOURCE: Pontigo-Loyola AP, et al. (2008). Dental fluorosis in 12- and 15-year-olds at high altitudes in above-optimal fluoridated communities in Mexico. Journal of Public Health Dentistry 68(3):163-6.

Iodine Deficiency in the United States

Over the past few decades, the rate of iodine deficiency has increased in the United States. According to the National Research Council (NRC), “Iodine intake in the United States (where iodine is added to table salt) is decreasing, and an estimated 12% of the population has low concentrations of urinary iodine.” (NRC 2006). In light of this trend, the NRC has called upon researchers to begin studying the endocrine and neurological effects that fluoride exposures may be having on the health of people with low iodine intake. As the NRC stated in 2006:

“The effects of fluoride on various aspects of endocrine function should be examined further, particularly with respect to a possible role in the development of several diseases or mental states in the United States. Major areas for investigation include the following: thyroid disease (especially in light of decreasing iodine intake by the U.S. population).”