In terms of selenium toxicity, I have an acquaintance who was able to put his
incurable form of cancer into remission by taking
very high doses of sodium selenite, a form of selenium that has potent anticancer effects. This acquaintance took a dose of 5,000 mcg of selenium as sodium selenite each day for two years, with no sign of any selenium toxicity, apart from some nausea when he first started on this dose. This individual told me of many other cancer patients who treat incurable or chemotherapy-resistant cancers using very high dose sodium selenite, and none have any side effects other than some nausea.
Note that the absorption of sodium selenite in the gut is only around 50%, so a 5,000 mcg dose of selenium as sodium selenite would be equivalent to 2,500 mcg of selenomethionine, as the latter has 100% absorption.
By the way, if anyone wants more information about this intriguing sodium selenite protocol for cancer, I posted some details
here.