"I do not know for certain what the true overdose is. I have heard 400 mcg, but that varies also depending on the article.

Overdose of selenium could also vary from person to person depending on what their needs are."

Of course. Sometimes for certain substances they talk about toxicity in mcg or mg per kilogram of body weight, so the toxic amount would be double for a 200 pound man that it is for a 100 pound woman for example. Not all substances work like that though. For many things though, they seem to give a guideline based on a 150 pound person.