“Most experimental studies in animals designed to characterize the effects of perchlorate exposure have been done in rats. However, rats are much more sensitive to agents that disturb thyroid function than are humans, so the relevance of rat studies in quantitative terms to humans is limited.” (p 100)
“Humans are much less susceptible than rats to disruption of thyroid function and therefore are not likely to develop thyroid tumors as a result of thyroid exposures.” (p 109)
“The committee considered several of the animal studies on which EPA based its point of departure to be flawed in their design and execution. Conclusions based on those studies, particularly the neurodevelopmental studies, were not supported by the results of the studies.” (p 112)
“The committee reviewed the human and animal data and found that the human data provided a more reliable point of departure for the risk assessment than the animal data.” (p 113)
U.S. EPA’s Policy for Assessment of Thyroid Follicular Tumors states that “rodents show significant increases in cancer with thyroid pituitary disruption; humans show little, if any.”
(http://epa.gov/osp/spc/spcthyr.htm) In other words, the rat is more sensitive to thyroid-pituitary disruption than humans.
There are several reasons why rats are considered more susceptible to thyroid disruption than humans:
For one, humans and rodents have different proteins that attach to and carry thyroid hormones to different sites in the body where the hormones are used.
The protein found in humans binds the thyroid hormones more tightly, so that the hormones stay in the body much longer.
Because the turnover of thyroid hormones in rodents is much greater than in humans, the rodent thyroid must produce much more thyroid hormone.
To accomplish this, rodents produce more thyroid stimulating hormones, which in turn has a greater stimulatory effect on the thyroid.