Deficiency, although relatively rare in healthy well-nourished individuals,[1] can have significant negative results,[2] affecting the health of the heart and the nervous system; contributing to depression, anxiety, and dementia; and interfering with reproduction and gestation.

Selenium is also necessary for the conversion of the thyroid hormone thyroxine (T4) into its more active counterpart triiodothyronine (T3),[3] and as such a deficiency can cause symptoms of hypothyroidism, including extreme fatigue, mental slowing, goiter, cretinism, and recurrent miscarriage.

[5] It can occur in patients with severely compromised intestinal function, those undergoing total parenteral nutrition, those who have had gastrointestinal bypass surgery, and also in persons of advanced age (i.e., over 90).

[citation needed] Increased risk for developing various diseases has also been noted, even when certain individuals lack optimal amounts of selenium, but not enough to be classified as deficient.

[citation needed] For some time now, it has been reported in the medical literature that a pattern of side effects possibly associated with cholesterol-lowering drugs (e.g., statins) may resemble the pathology of selenium deficiency.

[7][8] The European Food Safety Authority (EFSA) recommends a dietary allowance of 70 μg per day selenium intake for adults.

In some regions (e.g. much of the northeastern and northwestern US and adjacent Canada, and the southeastern US), selenium deficiency in some animal species is common unless supplementation is carried out.