The Brazil nut is probably the best source of selenium out there. Liver and fish also have high levels of selenium, but nothing that can match the nutritional punch that Brazil nuts pack.
Selenium is a trace mineral found in a variety of foods and necessary for, among other things, a healthy thyroid. Selenium helps to activate and then deactivate various thyroid hormones and their metabolites. As a result, selenium appears to play a role in inhibiting Hashimoto’s disease, a condition in which the body attacks its own thyroid cells.
In the case of Brazil nuts, a little definitely goes a long way. Like most nuts, Brazil nuts contain an anti-nutrient called phytic acid, which binds to minerals, prevents their absorption into the bloodstream, and causes digestive issues in some individuals. Brazil nuts are also high in radium so eating more than a handful at a time could potentially be dangerous.