The suggested upper limit for selenium is 200 mcg per day.

Selenium with zinc are cofactors in thyroid hormone conversion to T3 in tissues.
So we really need it.

However, just recently I saw a paper, that stated high selenium intake is
associated with developing Type II diabetes!

http://www.news.cornell.edu/Chronicl..._diabetes.html

http://care.diabetesjournals.org/cgi...tract/30/4/829

So I am not sure where we should go with selenium supplements.
You could try just eating some selenium rich foods if you think you are a risk for diabetes.

Here is a good monograph:
http://lpi.oregonstate.edu/infocente...ium/index.html

There has been much research on the benefits and necessity for selenium,
that a compromise should be done I think. And I have read that modern Brazil nuts grown in poor soils, have lower amounts.

Selenium is also thought to sequester mercury---and this may be important for some people:
http://www.mercuryfacts.org/fSelenium.cfm

So the selenium question is complex.