Hey there! I've had big differences in reactions to selenium. Last year I took it at about 300 mcg per day and got slow reactions in sight - when looking from bright areas to dark areas, my irises were slow in reacting.
These past weeks, I have been using it here and there in 200 mcgs, with more of it's co-factors. Also, I've read that using TMG relieves a lot of stress on the liver and allows one to absorb and use selenium a lot better.
I use these cofactors:
b-complex
zinc
magnesium malate and chloride
vitamin D3
Basically selenium-high foods have all the co-factors present needed to allow the body to use the selenium properly- so it may be in your best interest to stick to the food-sources!