Hi greenmom, welcome to the forum!
We take 200 mcg of selenium everyday, in addition to what's in our daily multiple. Included in all the benefits, is helping to avoid dementia. Here's some additional info.
Selenium is an essential trace element for humans and other animals. Selenium was named after the moon goddess, Selene, by the Swedish chemist Jons Jakob Berzelius in 1817. Dr. Klaus Schwarz established selenium as an essential nutrient for animals in 1957, but the first selenium function in humans wasn't discovered until 1973. 
Dr. John Rotruck and his colleagues at the University of Wisconsin demonstrated that selenium was incorporated into molecules of an enzyme called glutathione peroxidase (GPX). This vital enzyme protects red blood cells, cell membranes and sub-cellular components against undesirable reactions with soluble peroxides.
The discovery of GPX opened the door to our understanding of how selenium is protective against cancer, heart disease, arthritis and accelerated aging. Now more scientific excitement is being generated with the recent finding that selenium is also a vital component of other mammalian enzymes.
Phospholipid Hydroperoxide Glutathione Peroxidase (PHGPX) protects membranes against peroxides already bound to membrane surfaces. 
PHGPX blocks formation of the extremely harmful alkoxyl radical and inhibits peroxidative chain branching. This activity is even of more importance in the prevention of cancer, heart disease and accelerated aging.
Now addition pathways in which selenium is involved in health are being uncovered. Selenium is a component of the enzyme that is needed to produce the most active thyroid hormone. Sub-optimal amounts of selenium impair thyroid hormone function and thus affects many body functions.
Biochemists are now studying several other selenium proteins and have classified them into four main categories. 
First there were animal studies that showed that selenium protected against chemicals and ultraviolet energy that cause cancer. [4-7]
These laboratory findings were also supported by epidemiological studies (population surveys), and now large scale clinical studies are being sponsored by the U. S. government. [8-15]
Epidemiological studies have shown that persons with low-selenium diets have two-to-three times greater risk of heart disease than those eating selenium rich diets. 
In a clinical study, patients with blockage of all three coronary arteries had low blood selenium levels, while those with high blood selenium levels were healthy and free of coronary heart disease. 
Strikingly, those with one diseased coronary artery had the next highest blood selenium levels, and those with two blocked coronary arteries had the second lowest blood selenium levels.
Clearly, the antioxidant protection of the selenium-containing enzymes, GPX and PHGPX, protect the arteries and cholesterol-carrying lipoproteins against the damage that leads to heart disease.
Arthritic inflammation is produced by certain hormone-like compounds called prostaglandins. Selenium is involved in controlling these specific prostaglandins by controlling the free-radical damage that stimulates their production.
Norwegian physicians had noted that arthritis patients tended to have low blood selenium levels. When their arthritis ***patients were given selenium supplements, they dramatically** improved. 
A Danish study has confirmed the Norwegian study. 
Selenium is a component of the enzyme that is needed to produce the thyroid hormone triiodothyronine (T3). T3 is the preponderant metabolic thyroid hormone. The selenium-containing enzyme, iodothyronine deiodinase, converts the prohormone thyroxine (T4) into T3. 
This explains the observation that selenium deficiency impairs thyroid hormone function. Impaired thyroid hormone function is called hypothyroidism and affects many body functions.