Excellent Q. I am aware the selenium overdose occurs, most commonly reported for folks taking large volumes of supplements over long periods of time. One can get endocrine disturbances, especially thyroid issues. I think one could run into trouble at 400 mcg daily or more long-term. I personally would stick to one or two Brazil nuts daily, even as I recognize there is variability in the amount any given nut contains. If I had a diet high in selenium, I might even skip the Brazil nuts. THank you for this reminder.