Thyroid Health and Fluoride Exposure
“The effects of fluoride on various aspects of endocrine function should be examined, particularly with respect to a possible role in the development of several diseases or mental states in the United States. Major areas of investigation include . . . thyroid disease (especially in light of decreasing iodine intake by the U.S. population).” (National Research Council 2006).
The Thyroid
The thyroid gland regulates the body’s metabolic rate and plays an exquisitely important role in human health. Because all metabolically active cells require thyroid hormone for proper functioning, thyroid disruption can have a wide range of effects on virtually every system of the body. Chemicals that interfere with thyroid function must be treated with great caution.
According to the U.S. National Research Council, and as discussed below, there is substantial evidence that fluoride exposure can impact thyroid function in some individuals. (NRC 2006). There have been three human IQ studies that have taken the thyroid into account:
- Wang M, et al. (2019). Thyroid function, intelligence, and low-moderate fluoride exposure among Chinese school-age children. Environment International 134:105229
- Zhang S, et al. (2015). Modifying Effect of COMT Gene Polymorphism and a Predictive Role for Proteomics Analysis in Children’s Intelligence in Endemic Fluorosis Area in Tianjin, China. Toxicological Sciences 144(2):238-45. April.
- 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.
Fluoride as an Anti-Thyroid Agent
When people think of fluoride being prescribed for medicinal purposes, they generally think of fluoride supplementation to reduce tooth decay. Fluoride, however, has also been prescribed as a drug to reduce the activity of the thyroid gland.
Up through the 1950s, doctors in Europe and South America prescribed fluoride to reduce thyroid function in patients with over-active thyroids (hyperthyroidism). (Merck Index 1968). Doctors selected fluoride as a thyroid suppressant based on findings linking fluoride to goitre, and, as predicted, fluoride therapy did reduce thyroid activity in the treated patients. (McClaren 1969; Galletti 1958; May 1937). Moreover, according to clinical research, the fluoride dose capable of reducing thyroid function was notably low — just 2 to 5 mg per day over several months. (Galletti & Joyet 1958). This dose is well within the range (1.6 to 6.6 mg/day) of what individuals living in fluoridated communities are now estimated to receive regularly. (DHHS 1991).
Fluoride And Hypothyroidism
Based on fluoride’s anti-thyroid effects in hyperthyroid patients, concerns have arisen about whether current fluoride exposures could be contributing to the increased prevalence of under-active thyroid (clinical and/or subclinical hypothyroidism) in the United States and other nations. 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).” (Peckham 2015).
Supporting the fluoride/hypothyroidism connection are a number of studies from China, India, and Russia that have found alterations in thyroid hormones, including reduced T3 and increased TSH, in populations exposed to elevated levels of fluoride in the workplace or in the water. (NRC 2006; Susheela 2005; Mikhailets 1996; Yao 1996; Bachinskii 1985; Yu 1985).
In clinical hypothyroidism, the thyroid gland fails to produce sufficient quantities of the hormones triiodothyronine (T3) and thyroxine (T4). These hormones are required by all metabolically active cells, and their reduced presence can thus produce a range of ill effects, including fatigue, muscle/joint pain, depression, weight gain, menstrual disturbances, impaired fertility, impaired memory, and inability to concentrate. When T3 and T4 levels begin to fall, the pituitary gland responds by increasing production of “Thyroid Stimulating Hormone” (TSH) as a means of getting the thyroid to produce more T3 and T4.
In subclinical hypothyroidism, the TSH level is elevated, but the T3 and T4 hormones are still within the normal range. Although subclinical hypothyroidism used to be regarded as largely inconsequential, it is increasingly considered a “clinically important disorder.” (Gencer 2012).
Some studies have found, for example, that subclinical hypothyroidism in pregnant women results in reduced IQ in offspring, (Klein 2001; Haddow 1999), and a recent study in the Journal of the American Medical Association found that adults with subclinical hypothyroidism had a significantly higher rate of coronary heart disease. (Rodondi 2010).
Studies investigating fluoride’s impact on thyroid hormone levels have produced divergent findings, but are consistent with fluoride having an anti-thyroid effect under certain circumstances. (NRC 2006). The most common thyroid effect associated with fluoride exposure appears to be an increase in TSH levels, with or without a corresponding effect on T3 or T4. (Susheela 2005). One of the most recent studies, for example, found a trend towards higher TSH in children based on the severity of their dental fluorosis, but without a significant effect on either T3 or T4. (Hosur 2012, see figure).
These and other findings indicate that fluoride can contribute to a subclinical, if not clinical, hypothyroid condition. It remains difficult to predict the toxic dose, however, as it appears to depend, in part, on the nutritional and health status of the individual, particularly the adequacy of iodine intake. (NRC 2006).
Fluoride Exacerbates the Impact of Iodine Deficiency
A consistent body of animal and human research shows that fluoride exposure worsens the impact of an iodine deficiency. (Gas’kov 2005; Hong 2001; Wang 2001; Zhao 1998; Xu 1994; Lin 1991; Ren 1989; Guan 1988). 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. (Iodine deficiency is the leading cause of mental retardation throughout the world.)
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. (Hong 2001; Xu 1994; Lin 1991; Ren 1989). The studies, which utilize childhood intelligence as the metric for assessing neurological health, have found that fluoride levels as low as 0.9 ppm can worsen the IQ effect of iodine deficiency. (Lin 1991). This concentration is within the purported “optimal” range of fluoride that is added to water in water fluoridation programs (0.7-1.2 ppm).
While many studies have found an association between fluoride and reduced IQ among children with adequate iodine intake, (Choi 2012), an iodine deficiency will lower the threshold at which fluoride damages the brain. (Xu 1994; Guan 1988). An iodine deficiency will also lower the threshold for other forms of fluoride toxicity as well, including dental fluorosis. (Zhao 1998; see also Pontigo-Loyola 2008).
Iodine Deficiency Remains a Public Health Concern in the U.S.
Despite the widespread availability of iodized salt, iodine deficiency has re-emerged as a public health concern in the United States. (CDC 1998). More than 11% of all Americans, and more than 15% of American women of child-bearing age, presently have urine iodine levels less than 50 mcg/L (Caldwell et al., 2008), indicating moderate to severe iodine deficiency. An additional 36% of reproductive-aged women in the U.S. are considered mildly iodine deficient (<100 mcg/L urinary iodine). Fluoride’s ability to worsen the effects of an iodine deficiency could thus be highly relevant to populations in the U.S. The National Research Council has therefore called upon the scientific community to begin investigating the interactive effects of fluoride and iodine in U.S. populations. So far, no such research has been conducted.
Fluoride And Goitre
Studies dating back to the 19th century have implicated fluoride as a possible cause of goitre. Goitre (aka goiter) is an enlargement of the thyroid gland that in some cases can produce visible swelling in the neck. Although the main cause of goitre is iodine deficiency, it can also be caused by other things, including hypothyroidism and goitrogens (substances that cause goitre).
Studies that have examined human populations with adequate intake of iodine have reported mixed results about fluoride’s ability to produce goitre. (NRC 2006; Burgi 1984; McLaren 1969). The research has been more consistent, however, where the examined populations had either excessive iodine intakes, or deficient iodine intakes. (Gas’kov 2005; Hong 2001; Wang 2001; Xu 1994; Yang 1994; Lin 1986). Most of this latter research was initially published in either Russian or Chinese and was only recently translated into English by the Fluoride Action Network. Accordingly, previous reviews of fluoride/goitre research (e.g., NRC 2006) were not able to take these studies into account. As such, the evidence linking fluoride to goitre for populations with excessive, or deficient, iodine exposure is stronger than previously recognized. Read more.
Fluoride’s Potential Effects on Dogs
An investigation by the Environmental Working Group found that commercial dog food contains very high levels of fluoride (due, in part, to the presence of fluoride-rich bone particles). Since dogs have been found to suffer a high incidence of hypothyroidism, the relationship between fluoride contamination and thyroid disease in dogs deserves further attention, particularly since it was fluoride’s production of goiter in dogs that first prompted the idea that fluoride could be an anti-thyroid agent. (Maumene 1854).
Fluoride and the Pineal Gland
In the 1990s, a British scientist, Jennifer Luke, discovered that fluoride accumulates to strikingly high levels in the pineal gland. (Luke 2001). The pineal gland is located between the two hemispheres of the brain and is responsible for the synthesis and secretion of the hormone melatonin. Melatonin maintains the body’s circadian rhythm (sleep-wake cycle), regulates the onset of puberty in females, and helps protect the body from cell damage caused by free radicals.
While it is not yet known if fluoride accumulation affects pineal gland function, preliminary animal experiments found that fluoride reduced melatonin levels and shortened the time to puberty. (Luke, 1997). Based on this and other evidence, the National Research Council has stated that “fluoride is likely to cause decreased melatonin production and to have other effects on normal pineal function, which in turn could contribute to a variety of effects in humans” (NRC, 2006, p. 256).
The Pineal Gland Has Highest Levels of Fluoride in Body
As a calcifying tissue that is exposed to a high volume of blood flow, the pineal gland is a major target for fluoride accumulation in humans. In fact, the calcified parts of the pineal gland (hydroxyapatite crystals) contain the highest fluoride concentrations in the human body (up to 21,000 ppm F), higher than either bone or teeth (Luke 1997; 2001). Although the soft tissue of the pineal does not accumulate fluoride to the same extent as the calcified part, it does contain higher levels of fluoride than found than in other types of soft tissue in the body, with concentrations (~300 ppm F) that are known in other contexts to inhibit enzymes. While the impacts of these fluoride concentrations in the pineal are not yet fully understood, studies have found that calcified deposits in the pineal are associated with decreased numbers of functioning pinealocytes and reduced melatonin production (Kunz et al., 1999) as well as impairments in the sleep-wake cycle. (Mahlberg 2009).
Fluoride and Earlier Puberty in Girls
In the United States, children are reaching the age of puberty at earlier ages than in the past, a trend that carries health consequences, including a heightened risk for breast cancer. Some evidence indicates that fluoride, via its effect on the pineal, could be a contributing cause to this trend.
In animal studies, for example, fluoride exposure has been found to cause a decrease in the amount of circulating melatonin and lead to an accelerated sexual maturation in females. (Luke 1997). Similar findings have been reported in two epidemiological studies of human populations drinking fluoridated water. In the first published fluoridation safety experiment in Newburgh, New York, the authors found that girls living in a fluoridated community reached puberty five months earlier than girls living in a non-fluoridated community. (Schlesinger 1956) Later, in 1983, Farkas reported that postmenarcheal girls were “present at younger ages in the higher fluoride town than in the low-fluoride town, although the reported median ages were the same.”
Male Fertility and Fluoride Exposure
Male infertility is responsible for about 50% of the fertility problems that couples face. Infertility in males is often the result of reduced sperm court, abnormal sperm quality (e.g., reduced motility and altered morphology), or altered levels of sex hormones (e.g., reduced testosterone). A review of over 100 studies of sperm density from 1938 to 1996 found that human sperm count has significantly declined in North America and Europe since the 1940s (Swan 2000). While the causes of this decline are not entirely known, fluoride exposure, particularly from high-concentration topical fluoride gels, must be considered as one of the potential contributing factors.
Fluoride Affects Sperm Quality
In 2002 and again in 2006, researchers from Poland reported that exposing ram semen to 0.38 parts per million (20 umol/L) of fluoride for 5 hours was sufficient to “cause a statistically significant decrease in the motility of spermatoza and the number of intact acrosomes.” (Zakrzewska 2002). As the authors noted, these changes would “undoubtedly affect the physiological function of the sperm.” Prior to the Polish team’s findings, researchers from Texas found that infusing testis with higher, but still relatively modest, levels of fluoride (4.75 ppm) “unequivocally” inhibited the synthesis of testosterone. (Chubb 1985). Read More.
The Polish team’s findings are of particular importance when considering that from the 1960s to the 1990s, the use of high-concentration topical fluoride gels produced blood concentrations in boys and men that far exceeded 0.38 ppm. In tests on both children and adults, the use of topical fluoride gels at the dental office has been found to produce blood fluoride concentrations as high as 1.2 ppm, or four times higher than the concentration found to damage sperm. (Ekstrand 1980, 1981). Further, the blood fluoride concentrations have been found to exceed 0.38 ppm for up to six hours after treatment (longer than the length of time that the Polish researchers exposed the semen). Although most dentists now use precautionary procedures (e.g., suction devices) to reduce blood fluoride concentrations following application of fluoride gels, available data shows that children will still routinely ingest enough fluoride from topical gels to reach blood fluoride concentrations exceeding 0.38 ppm.
Fluoride’s Impact on the Male Reproductive System in Animal Studies
Consistent with the in vitro research, over 60 animal studies have found that fluoride adversely impacts the male reproductive system. The effects, which have been observed in rats, mice, chickens, and rabbits, include: (1) decreases in testosterone levels; (2) reduced sperm motility; (3) altered sperm morphology; (4) reduced sperm quantity; (5) increased oxidative stress; (6) and reduced capacity to breed.
While the studies have generally used high doses, many of the studies have found effects at dosages that would produce blood fluoride concentrations far lower than the concentrations used in the in vitro research. See, e.g., Sun (2010); Dvoráková-Hortová (2008); Sharma (2008); Reddy (2007); Gupta (2007); Pushpalatha (2005). In one of the few studies to report blood fluoride concentrations, Mexican researchers reported that blood fluoride levels of 0.2 to 0.26 ppm for an eight week period caused increased oxidative stress, reductions in sperm motility and reduced fertility in male rats. Izquierdo-Vega, et al. (2008).
While some studies have not found any effects of high fluoride dosages on the reproductive functions of male rats , these studies represent the distinct minority in the field. (Sprando & Collins 1996, 1997, and 1998). One possible explanation for the discrepancy in findings is the nutritional health of the tested animals. As with many other areas of fluoride research, nutritional deficiencies (e.g., protein) unequivocally exacerbate fluoride’s reproductive effects, whereas nutritional supplementation (e.g., protein or anti-oxidants such as vitamin C) can significantly prevent or ameliorate these effects.
High Fluoride Exposure Linked to Reduced Testosterone and Decreased Fertility in Humans
Consistent with the in vitro and animal research, studies of human populations have reported associations between fluoride exposure and damage to the male reproductive system. Most notably, a scientist at the Food & Drug Administration reported in 1994 that populations in the United States with more than 3 ppm fluoride in their water had lower “total fertility rates” than populations with lower fluoride levels. (Freni 1994). While 3 ppm is a higher concentration than used in water fluoridation programs (0.7 to 1.2 ppm), it is still considered a “safe” level by the EPA.
To date, no U.S. health agency has attempted to replicate Freni’s findings. However, three studies of highly fluoride-exposed populations in China and India have found that high fluoride exposure is associated with reduced male fertility. (Chen 1997; Liu 1988; Neelam 1987). In addition, five studies from China, India, Mexico, and Russia have found that high-fluoride exposure is associated with reduced male testosterone levels. (Hao 2010; Ortiz 2003; Susheela 1996; Michael 1996; Tokar 1977).
Fluoride And The Pancreas
The pancreas is the gland that produces insulin, a hormone that regulates the uptake of glucose from the bloodstream. Animal and human studies have repeatedly found that fluoride exposure can increase the levels of glucose in the blood. Chronic elevation of blood glucose levels is the hallmark of Type II Diabetes. While the mechanism underlying fluoride’s effect on glucose levels has yet to be determined, some research suggests it could be the result of fluoride’s impact on the quantity of, or cellular responsiveness to, the insulin produced. Read more.
Fluoride And The Parathyroid Gland
The parathyroid gland produces parathyroid hormone (PTH). PTH regulates the amount of calcium in our bones and blood supply. When the calcium level in blood starts to fall, PTH triggers the breakdown of bone tissue to transfer the body’s stored supply of calcium into the blood supply. When the parathyroid produces too much PTH a condition known as hyperparathyroidism develops. Hyperparathyroidism has been found to occur as a secondary effect of the fluoride-induced bone disease skeletal fluorosis and may help to explain some of the bone effects encountered in fluorosis. Read more.