In my opinion, it hasn't quite been proven that artificial fluoridation causes thyroid issues, but it is far more likely than not that it does.
Here are some quotes from the 2006 US National Research Council report titled Fluoride in Drinking Water: A Scientific Review of EPA's Standards.
p 189 "The endocrine system, apart from reproductive aspects, was not considered in detail in recent major reviews of the health effects of fluoride (PHS 1991; NRC 1993; Locker 1999; McDonagh et al. 2000a; WHO 2002; ATSDR 2003)." The 2007 Australian NHMRC report can be added to that list.
p 189 "an estimated 12% of the population has low concentrations of urinary iodine"
p 197 "As with the animal studies, high fluoride intake appears to exacerbate the effects of low iodine concentrations. Uncertainty about total fluoride exposures based on water fluoride concentrations, variability in exposures within population groups, and variability in response among individuals generally have not been addressed."
p 197 "several lines of information indicate an effect of fluoride exposure on thyroid function. However, because of the complexity of interpretation of various parameters of thyroid function, the possibility of peripheral effects on thyroid function instead of or in addition to direct effects on the thyroid, the absence of TSH measurements in most of the animal studies, the difficulties of exposure estimation in human studies, and the lack of information in most studies on nutritional factors (iodine, selenium) that are known to affect thyroid function, it is difficult to predict exactly what effects on thyroid function are likely at what concentration of fluoride exposure and under what circumstances"
p 197 "Several sets of reported results are consistent with an inhibiting effect of fluoride on deiodinase activity; these effects include decreased plasma T3 with normal or elevated T4 and TSH and normal T3 with elevated T4… The antihyperthyroid effect that Galletti and Joyet (1958) observed in some patients is also consistent with an inhibition of deiodinase activity in those individuals."
p 198 "Subclinical hypothyroidism is considered a strong risk factor for later development of overt hypothyroidism. Biondi et al. (2002) associate subclinical thyroid dysfunction… with changes in cardiac function and corresponding increased risks of heart disease.… subclinical hypothyroidism is associated with increased cholesterol concentrations, increased incidence of depression, diminished response to standard psychiatric treatment, cognitive dysfunction, and, in pregnant women, decreased IQ of their offspring"
p 198 "The possibility that either dental fluorosis (Chapter 4) or the delayed tooth eruption noted with high fluoride intake… may be attributable at least in part to an effect of fluoride on thyroid function has not been studied."
p 218 "In humans, effects on thyroid function were associated with fluoride exposures of 0.05-0.13 mg/kg/day when iodine intake was adequate and 0.01-0.03 mg/kg/day when iodine intake was inadequate"
