Brazil nuts do contain selenium which is an antioxidant so it's good for your health to eat Brazil nuts especially if you have a selenium deficiency.
Selenium is necessary for the conversion of the thyroid hormone thyroxine.
When you have a deficiency of Selenium it can cause symptoms of hypothyroidism, including extreme fatigue, mental slowing, goiter, cretinism, and even miscarriage.
Brazil nuts are also a good source of and contain protein, fiber, vitamin E, thiamin, iron, magnesium, and zinc.
However be aware that if you eat more than 6 Brazil nuts per day you can get radiation poisoning so don't eat more than 6 per day to be safe.