An essential trace mineral, selenium is necessary for normal functioning of the immune system and thyroid gland, and helps protect cells against free radicals that can damage cells, contribute to chronic diseases, and promote cancers.
Selenium deficiency, common in areas with selenium-deficient soil, may lead to heart problems, gastrointestinal disorders, and may also affect thyroid function. Low levels of selenium may also be associated with an increased risk of rheumatoid arthritis. So, selenium levels should be tested in anyone who suspects occupational exposure (toxic levels) and monitored in those living in areas of selenium-deficient soil, as well as anyone who has a risk of (or who already has) heart disease, cancer, or arthritis.
(Source: Life Extension magazine, November 2003 issue.)