Up through the 1950s, doctors in Europe and South America prescribed fluoride for this purpose in patients with hyperthyroidism. (Merck Index 1968). Fluoride was selected as a thyroid suppressant based on findings dating back to the mid-19th century that fluoride is a goitrogen (a substance that can cause goiter). When used as a remedy for hyperthyroidism, daily treatments of just 2 to 5 mg/day fluoride ion were enough to reduce most patients’ thyroid activity. (Galletti & Joyet 1958).

Based on fluoride’s anti-thyroid effects in hyperthyroid patients, concerns have arisen about whether elevated fluoride exposures can impact thyroid activity in some members of the population, particularly those with an iodine deficiency. In February 2015, British scientists reported that fluoridated water in Britain is associated with elevated rates of hypothyroidism:

“We found that higher levels of fluoride in drinking water provide a useful contribution for predicting prevalence of hypothyroidism. We found that practices located in the West Midlands (a wholly fluoridated area) are nearly twice as likely to report high hypothyroidism prevalence in comparison to Greater Manchester (non-fluoridated area).”
SOURCE: Peckham S, et al. (2015). Journal of Community Health & Epidemiology (see study)

Supporting the fluoride/hypothyroidism connection are a number of studies from China, India, and Russia that have found alterations in thyroid hormones in populations exposed to elevated levels of fluoride in the workplace or in the water. As discussed at length by the National Research Council’s 2006 review of the literature, studies have found varying associations between fluoride exposure and thyroid hormone status, including:

  1. normal T3/normal T4/elevated TSH
  2. reduced T3/normal T4/elevated TSH
  3. normal T3/elevated T4/elevated TSH
  4. normal T3/elevated T4/normal TSH
  5. no detectable alterations.

The reasons for these differing responses are not yet entirely clear. One possibility is that fluoride’s anti-thyroid effects are context-dependent, with effects differing based on the nutritional and health characteristics of the individual or population being studied. It is known, for example, that fluoride’s effect on thyroid function and neurological health is significantly more severe in populations with an iodine deficiency, and to a lesser extent iodine excess, than in populations with adequate iodine intake. According to a 2006 review by the National Research Council, the evidence indicates that fluoride does impair thyroid function, but it remains “difficult to predict” at what concentrations, and under what circumstances, this effect(s) occurs.

Fluoride, Elevated TSH, and Reduced IQ:

Perhaps the most consistent finding in the fluoride/hormone studies thus far is the association between fluoride and elevated TSH. According to the NRC, TSH is considered a “’precise and specific barometer’ of thyroid status in most situations.” The relationship between fluoride and elevated TSH has been found even where T3 and T4 levels remain normal, thus suggesting that fluoride could contribute to subclinical hypothyroidism, which is a condition of “mild thyroid failure” marked by increased TSH and normal T3/T4.

While subclinical hypothyroidism used to be regarded as largely inconsequential, it is now increasingly considered a “clinically important disorder that has adverse  clinical consequences.” (Gencer 2012). Several studies, for example, have found that subclinical hypothyroidism in pregnant woman was a risk factor for reduced IQ in the offspring. (Klein 2001; Haddow 1999). This is an interesting finding in light of the 40+ studies associating fluoride exposure with reduced IQ in children. Although most of these studies did not measure TSH, those that did so reported that children with high fluoride exposures had elevated TSH levels. (Wang 2001; Yao 1996; Lin 1991). Further, in the Lin study, the authors reported that elevated TSH correlated with reduced IQ. It appears possible, therefore, that fluoride’s effect on TSH levels could be one of the contributing factors towards the reduced IQ reported in the studies to date.

Another possible consequence of subclinical hypothyroidism is heart disease. In 2010, a study in the Journal of the American Medical Association found that adults with subclinical hypothyroidism had a significantly higher incidence of, and mortality from, coronary heart disease. (Rodondi 2010). Whether this could help explain the relationship between elevated fluoride and cardiovascular disease remains to be determined. As reported below, one recent study (Karademir 2011) did find a relationship between fluoride exposure, thyroid levels, and cardiovascular indices, although TSH levels were not found to be elevated.

Thyroid Hormone Levels Based on Severity of Dental Fluorosis. (Hosur 2012).

Studies on thyroid hormone levels as function of fluoride exposure:

“[T]hyroid hormones (T3 and T4) were poorly correlated with the concentration of water chemicals. TSH was significantly correlated with F– concentration (r=0.506, P=0.004) and inversely correlated with Ca2+ concentration (r=-0.506, P=0.004) and hardness (r=-0.506, P=0.004).”
SOURCE: Medani AM, et al. (2013). Excessive iodine intake, water chemicals and endemic goitre in a Sudanese coastal area. Public Health Nutrition 16(9):1586-92.

“Pairwise comparison of TSH values between mild and moderate and between mild and severe groups showed that TSH did not significantly vary between these groups, but a significant difference was found between TSH levels of moderate and severe groups.”
SOURCE: Hosur MB, et al. (2012). Study of thyroid hormones free triiodothyronine (FT3), free thyroxine (FT4) and thyroid stimulating hormone (TSH) in subjects with dental fluorosis. European Journal of Dentistry 6(2):184-90.

“In this study we examined the deleterious effect of fluorosis on cardiovascular system including detailed ECG with dispersion analysis, echocardiography, and HRV with Holter analysis in children. We found statistically significant low T4 levels, hypocalcemia and hyponatremia, increased QT and QTc interval in children with dental fluorosis. Our results show that fluorosis might increase risk of arrhythmia indirectly, due to its hypocalcemic, hypernatremic, and hypothyroidism effects.”
SOURCE: Karademir S, et al. (2011). Effects of fluorosis on QT dispersion, heart rate variability and echocardiographic parameters in children. Anadolu Kardiyol Derg 11(2):150-55.

“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.

“This study was designed to evaluate adverse health effects in adolescents from chronic exposure to various water fluoride concentrations in three communities located in Northern Mexico: Ciudad Juarez, Samalayuca, and Villa Ahumada. In these communities the fluoride concentration in water averages 0.3, 1.0, and 5.3 mg/L, respectively. The residents of Villa Ahumada have been exposed to excessive levels of fluoride in drinking water since their birth. . . . In Villa Ahumada, a significant inverse relationship was found between urine fluoride levels and stature; this association suggests that fluoride exposure may affect the teeth but also the growth of adolescents. Serum samples of these individuals showed elevated levels of alkaline phosphatase (ALP), potassium, magnesium, calcium, and phosphate, and decreased levels of thyroid hormone T3 and uric acid. These findings show that chronic exposure to high levels of fluoride have a definitive impact on the prevalence and severity of dental fluorosis, decreased stature, and decreased [] thyroid hormone secretion.”
SOURCE: Ruiz-Payan A. (2006). Chronic effects of fluoride on growth, blood chemistry and thyroid hormones in adolescents residing in three communities in Northern MexicoETD Collection for University of Texas, El Paso. Paper AAI3214004. http://digitalcommons.utep.edu/dissertations/AAI3214004

“In the current investigation 46.9% of the children in the [high fluoride] group have elevated TSH and normal FT4 and FT3 levels, while a similar derangement is also observed in 18.2% of the children in [the lower fluoride group]. This is our first category and is usually the first indication of thyroid dysfunction, termed sub-clinical hypothyroidism.”
SOURCE:  Susheela AK, et al. (2005). Excess fluoride ingestion and thyroid hormone derangements in children living in New Delhi, India. Fluoride 38(2):98-108.

“In this study, the serum levels of thyroxine (T4), triiodothyronine (T3), and protein-bound iodine (PBI) in the control cows were in the normal range of healthy cows, but they were significantly lower (p<0.05) in the fluorotic cows. These findings are consistent with the results of research with sheep, calves, cattle, and rats. . . . On the other hand, Choubisa reported that none of a group of fluorotic domestic animals exhibited any apparent evidence of hypothyroidism, stunted growth, [or] low milk production . . . . In our view, the reason for decreased levels of T4, T3, and PBI in our cows with chronic fluorosis might be due to: 1) inhibition of the absorption of the iodine and some amino acids (e.g., tyrosine) in the gastrointestinal tract, 2) insufficient synthesis and secretion of thyroglobulin and oxidized iodides from the thyroid glands, 3) low levels of bioavailable iodine in the Tendurek Mountain region.”
SOURCE: Cinar A, Selcuk M. (2005). Effects of chronic fluorosis on thyroxine, triiodothyronine, and protein-bound iodine in cows. Fluoride 38(1):65-68.

“TSH value was obviously higher than the control point, indicating that, under high iodine and high fluorine condition, T3 and T4 secreted by the thyroid are in the normal range, while TSH value secreted by the pituitary clearly increased. This is probably because high iodine and high fluorine suppress the synthesis and secretion of the thyroid peroxidase and thyroid hormones . . . .  The body accelerates the Hypothalamic TSH secretion by negative feedback regulation, thus increasing the secretion of TSH, stimulating the composition of T3 and T4 of the thyroid. As a result, the TSH in the peripheral blood circulation is high while T3 and T4 are not clearly reduced.”
SOURCE:  Wang X, et al. (2001). Effects of high iodine and high fluorine on children’s intelligence and thyroid functionChinese Journal of Endemiology 20(4):288-90.

[Objective: To study the significant test of diagnosing endemic fluorosis. Methods Twenty one routine and biochemical marks of blood and urine from 600 cases of the patients with different degree endemic fluorosis were determined and analysed. Results . . . The average of T3 and T4 were lower than the reference value, particularly in those with moderate and severe stages of the disease. Conclusions The RBC, Hb, serum calcium,phosphorus, AKP, urinary calcium, globulin, T3 and T4 were signifiant diagnostic indicators of endemic fluorosis.]
SOURCE: Wan G, et al. (2001). Determination and analysis on multimark of test of patients with endemic fluorosis. Chinese Journal of Endemiology 20(2):137-39.

[In a group of 8-12 year old children living in an endemic fluorosis area in China, TSH levels were significantly elevated, while T4 levels were significantly decreased and T3 levels significantly increased.]
SOURCE: Xiaoli L, et al. (1999). The detection of children’s T3, T4 and TSH contents in endemic fluorosis areas. Endemic Disease Bulletin 14(1):16-17.

“The TSH level is a sensitive index which both reflects the state of the body’s thyroid function, and screens the level of iodine (lack thereof) in a population. TSH is also a sensitive indicator in terms of making timely discoveries of people suffering from poor thyroid function or below-average intelligence. The results from this test show that TSH values of children with dental fluorosis from the two endemic areas is at a remarkably higher level than those from the non-endemic area. Children from the endemic areas were also found to have a lower level of intelligence than the non-endemic group. The heavier the level/concentration of fluoride found in the region, the more significant the difference in the results.” SOURCE: Yao Y, et al. (1996). Analysis on TSH and intelligence level of children with dental Fluorosis in a high fluoride area. Literature and Information on Preventive Medicine 2(1):26-27.

“Conclusions: 1. Abnormalities in the thyroid function characterized by a decreased iodine absorption function of the thyroid, a low level T3 syndrome, and a slight increase of the TSH level are observed in cases of chronic fluorine intoxication in the industrial workers. 2. The observed changes progressed with the increase of the time of exposure to fluorides and a more advanced disease stage. 3. The highest frequency of occurrence of the low level T3 syndrome was observed in workers with chronic fluoride intoxication including TPP (toxic liver damage). 4. The lowered iodine absorption function of the thyroid and/or the low level T3 syndrome can serve as diagnostic signs of chronic fluorine intoxication. 5. The decrease in the T3 level most probably occurs due to the disrupted conversion of T4 to T3 at the cell- target level. The disruption of conversion may be caused by fluorine affecting the enzyme system of deiodination as well as the toxic liver damage it causes.”
SOURCE: Mikhailets ND, et al. (1996). Functional state of thyroid under extended exposure to fluorides. Probl Endokrinol 42:6-9.

“The levels of serum T3, T4 and TSH were analyzed in children with fluoride-aluminum combined toxicosis in the Shuicheng area of Guizhou as compared with the children without fluoride-aluminum combined toxicosis. The results showed that serum T4 content decreased in the children with fluoride aluminum combined toxicosis (103.9±15.9 nmol/L vs 150.67±16.5 nmol/L, p 0 01), but no obvious differences of serum T3 and TSH were found among total three groups. It suggests that the disorder of the thyroid function should be considered when treating the children with fluoride aluminum combined toxicosis.”
SOURCE: Shufen J, et al. (1996). The change of thyroid functlon from children with fluoride aluminum combined toxicosis in Shuicheng area of Guizhou. Journal of Guiyang Medical College.

“While levels of thyroid stimulating hormone (TSH) and triiodothyronine (T3) did not vary, a significant increase in the thyroxine (T4) levels suggested alteration in thyroid function.”
SOURCE: Michael M, et al. (1996). Investigations of soft tissue function in fluorotic individuals of North Gujurat. Fluoride 29(2):63-71.

“Area A (high fluoride, low iodine) differed from area B (normal fluoride, low iodine) by having lower mean IQ, higher TSH, slightly higher 1311 uptake, and higher urinary iodine. . . . The significant ditferences in IQ among these regions suggests that fluoride can exacerbate central nervous lesions and somatic developmental disturbance caused by iodine deficiency. . . . [W]e found that 69% of the children with mental retardation had elevated TSH levels. IQ and TSH were negatively correlated. Many investigators regard an elevated TSH in the presence of normal T4 and T3 levels as evidence for hypothyroidism that is subclinical but that can still affect the development of brain and cerebral function to some degree.”
SOURCE: Lin F; et al (1991). The relationship of a low-iodine and high-fluoride environment to subclinical cretinism in XinjiangEndemic Disease Bulletin 6(2):62-67 (republished in Iodine Deficiency Disorder Newsletter Vol. 7(3):24-25).

“Endemic fluorosis is a systemic disease. We investigated the serum free fluoride, thyroid hormones and TSH concentrations in 37 cases. Significantly lowered serum T4 . . . and increased TSH were found in patients. Patients’serum T3 concentrations were not significantly different from the controls. Significant negative correlations were found between serum free fluoride concentrations and T3 concentrations or T3/T4 ratios. We propose that fluoride intoxication might decrease thyroid function and suggest the method to prevent and treat this condition.”
SOURCE: Liu Z, et al. (1988). An investigation on the serum thyroid hormones and fluoride concentrations in patients with endemic fluorosis. Chinese Journal of Endemiology 7(4):216-18. [Article in Chinese with English summary]

“The ingestion of drinking water with high concentrations of fluoride (122 +/- 5 micromoles per liter) leads, in healthy people, to stress of the functional status of the pituitary-thyroid system, as evidenced by a reduction in the concentration of T3, an increase  in the production (by the hypothalamus) of TSH in the serum, and a more avid uptake of I131 by the thyroid tissue. This permits us to classify the excessive accumulation of fluorine in the body as a risk factor providing a basis for the development of thyroid dysfunction.”
SOURCE: Bachinskii PP et al. 1985. Action of the body fluorine of healthy persons and thyroidopathy patients on the function of hypophyseal-thyroid the system. Probl Endokrinol (Mosk) 31(6):25-9. [Article in Russian, translated into English]

“A study on the serum T4, T3 and TSH levels was performed in 27 patients with chronic skeletal fluorosis and the data obtained were compared with those of 20 health persons. The results showed that serum T4 in the patients was lower than in the controls and TSH was higher, while serum T3 showed no significant difference. There was no goiter found in the patients. These data indicate that fluorine may reduce serum T4 by interfering [with] thyroid function.The increase of TSH secretion is the consequence stimulated by a feedback mechanism but no proliferation and enlargement of the thyroid gland resulted . . . .”
SOURCE: Yu Y. (1985). Study on serum T4, T3, and TSH levels in patients with chronic skeletal fluorosis. Chinese Journal of Endemiology 4(3):242-43.