An adequate iodine intake is essential for a proper functioning thyroid gland. When iodine intake is deficient, the thyroid is unable to produce sufficient quantities of the thyroid hormones T3 and T4 upon which countless systems in the body depend. Iodine deficiency disorders can thus wreak havoc on the body, including permanent brain damage if the deficiency exists during the early years of life. Excessive intake of iodine can also impair thyroid function, causing hypothyroidism and other thyroid disorders. (Leung 2012; Connelly 2012)

Just as fluoride can aggravate the impact of iodine deficiency, it may also aggravate the impact of iodine excess. In the following studies, Chinese researchers found that the combination of excess fluoride with excess iodine exacerbated the thyroid effects produced by either scenario in isolation. This research indicates that individuals with excessive iodine intake may be a previously overlooked part of the population with a heightened susceptibility to fluoride toxicity.

Human Studies:

“The concentration of serum TSH of children from high fluoride and iodine area and high iodine area was higher than that of children from high fluoride area and control area. Conclusion: High fluoride and iodine increase the prevalence of goiter. High iodine increases the concentration of FT4. Fluoride can increase the concentration of FT4 under high iodine condition.”
SOURCE: Ba Y, et al. (2009). Effect of different fluoride and iodine concentration in drinking water on children’s dental fluorosis and thyroid function. Chinese Journal of Public Health 25(8):942-43.

“In conclusion, high iodine and high fluorine in the drinking water have, to some extent, effects on children’s intelligence and thyroid function.”
SOURCE: Wang X, et al. (2001). Effects of high iodine and high fluorine on children’s intelligence and thyroid function. Chinese Journal of Endemiology 20(4):288-90.

“An excess of fluoride and a lack of iodine in the same environment has been shown to have a marked effect on child intellectual development, causing a more significant intellectual deficit than lack of iodine alone. In our study the study group of children from the high fluoride-high iodine village area had an average IQ of 76.67±7.75, which was somewhat lower than the control (IQ 81.67 ±11.9), although the difference is not statistically significant (P > 0.05). However, as seen in Table 2, the percentage of children in the low range (16.67%) is higher in the endemic group than in the control group (10.0%), suggesting that a high iodine-high fluoride environment also has a definite negative influence on child intellectual ability.”
SOURCE: Yang Y, et al. (1994). The effects of high levels of fluoride and iodine on intellectual ability and the metabolism of fluoride and iodine. Chinese Journal of Epidemiology 15(4):296-98 (republished in Fluoride 2008; 41:336-339).

“The number of children whose level of intelligence is lower is significantly increased in regions of high fluoride/iodine, regions of high fluoride only, regions of high fluoride/low iodine, against their respective comparative groups.”
SOURCE: Xu Y, et al. (1994). The effect of fluorine on the level of intelligence in children. Endemic Disease Bulletin 9(2):83-84.

 Animal Studies:

“OBJECTIVE: Investigating the influence of combined iodine and fluoride on phospholipid and fatty acid composition in brain cells of rats. METHODS: Five groups of rats were provided with deionized drinking water containing 0 and 150 mg/L NaF, and containing both 150 mg/L NaF and 0.003, 0.03 or 3 mg/L KI respectively for 5 months. Then phospholipid and fatty acid composition were determined using liquid chromatography. RESULTS: The phospholipid composition had no obvious change. The high concentration fluoride (150 mg/L) and high concentration Iodine (3 mg/L) with high concentration fluoride could cause significant changes of the fatty acid composition in brain cells of rats, the proportion of unsaturated fatty acid (C18:2) was significantly decreased and the saturated fatty acid (C12:0) increased obviously. The antagonistic action of 0.03 mg/L KI drinking water on this kind of influence induced by 150 mg/L NaF was the most evident, whereas that of 3 mg/L KI was action of synergetic toxicity. CONCLUSION: Fluorosis had obvious influence on phospholipid and fatty acid composition in brain cells of rats, and its mechanism might be associated with action of lipid peroxidation, and 0.03 mg/L KI is the optimal concentration for the antagonistic action with this influence from fluorosis.”
SOURCE: Shen X, et al. (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. [Article in Chinese]

“fluorine also affected the thyroid changes induced by ID [iodine deficiency] or IE [iodine excess]. After 100 days of treatment, fluorine showed some stimulatory effect on the thyroid in ID conditions and inhibitory effect in IE conditions. After 150 days, however, the effects of fluorine on the thyroid reversed as compared with that of 100 days. On the other hand, difference of iodide intake could also increase the toxic effects of FE on the incisors and bones.”
SOURCE: Zhao W, et al. (1998). Long-term effects of various iodine and fluorine doses on the thyroid and fluorosis in mice. Endocrine Regulation 32(2):63-70.

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