I agree with @mporaneos. My doctor tested me for a variety of vitamin and mineral levels to determine what I needed. Retesting is also important after 6-12 months to see if what you are taking is working and if adjustments need to be made to the doses of each you are taking, especially as you make better food choices for the vitamin and minerals you need. Sometimes too much is also not a good thing either.
One of the things I did for selenium is eating Brazil nuts - 3 to 4 each morning.