In the United States, iodine has been added to salt. However, as too much of salt intake leads to high blood pressure, health conscious people tend to limit the salt intake, which in return leads to iodine deficiency.
Furthermore, fluorine is added for disinfection in tap water. Fluorine, belonging to the same halogen group, interferes with the absorption of iodine in body, causing further iodine deficiency. And generally American people don't take sea weeds that contain lot of iodine. Thus, if you live in the United States, and do not eat sea foods, you might be at a risk of iodine deficiency.
Although being a vegetarian is generally good for health, if you like too much salad of cruciferous vegetables such as cauliflower, broccoli, cabbage, brussels sprouts etc., be cautions. These vegetables contain "Goitrogens" that interfere with iodine uptake in the thyroid gland. "Goiter" refers to the abnormal enlargement of the thyroid gland.
And because indole-3-carbinol, which is found at relatively high levels in cruciferous vegetables, is said to be effective to prevent breast cancer and prostate cancer (and lupus), some health conscious people eat these vegetables too much without knowing the danger of the adverse effect against the thyroid.
And people think any soy products are healthy. But if soy is not fermented, it can be hazardous to the thyroid, if taken too much. Many mothers feed soy milk to babies who have allergies to normal milk. Please search for "Soy Thyroid" in Google. You would read, drinking soy milk sometimes is good for health but when it comes to everyday, it might be dangerous.
In addition to this indiscreet soy intake, some people are taking vitamin C supplement too much. Recently, I saw a young girl nibbling on vitamin C tablets with crunching sound as if she was eating a cookie. This is very dangerous. In my 38 years of experience as a general physician, I understand vitamin C is certainly an important vitamin. However, it has been found recently that excess intake might lead to serious problems. Excess intake of vitamin C supplement lowers zinc and manganese level. And it inhibits the uptake of inorganic selenium. Zinc, manganese, and selenium are essential for the thyroid. So, if possible it is better to get vitamin C from food source rather than taking supplements recklessly.
Zinc is required for healthy T3 receptors in your cells so even if you have enough T3, it won't work optimally if you are deficient in Zinc.
Manganese is a vital component of the antioxidant enzyme called “MnSOD” (manganese-dependent superoxide dismutase). On liver cell membranes most T4 convert to T3, and there MnSOD is required to reduce oxidant stress.
Selenium is a vital component of the antioxidant enzyme “glutathione peroxidase” that has 8 isozymes. And 4 of the isozymes have selenium. Therefore to reduce H2O2 selenium plays a pivotal role. And selenium is essential to the 3 important enzymes to activate and deactivate T4 -- iodothyronine de-iodinases, types I, II, and III (known as IDI, IDII, and IDIII). All of these enzymes require selenium to function.
Humans and chimpanzees cannot make vitamin C in their liver as they lack gulonolactone oxidase. However, other mammals such as dogs, cats, lions etc. can synthesize vitamin C in their liver by this enzyme. That's why we don't see a lion eating oranges. As we cannot synthesize vitamin C inherently, we compensate it from the food sources.
If vitamin C is so important, why did humans and chimpanzees lose the ability to synthesize vitamin C in the body? From the evolutionary point of view, they should still have been equipped with it. Because the more evolved species should have the more perfect physiological functions. Some scientists feel that this is one of the defects of Darwin's theory of evolution.
But if excess vitamin C might give negative influence to the development of intelligence, we could understand why humans and chimpanzees have thrown away the genes encoding gulonolactose enzyme. If too much vitamin C interferes with absorption of the vital minerals such as zinc, manganese, and selenium, it hinders the thyroid gland from proper functioning and consequently the brain would not develop fully.
Both the soy products and supplements of vitamin C, looking like a healthy combination at first glance, probably affect the thyroid gland adversely if taken excessively, resulting in lowered function of the thyroid gland.
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