Selenium
An antioxidant trace element essential for thyroid and immune function, whose intake depends heavily on how rich the soil is.
Selenium is a component of major antioxidant enzymes (glutathione peroxidase) and plays a part in converting thyroid hormones (T4 → active T3) and in immunity. Needs are small but the margin is narrow.
The original angle (the geological mineral): a unique feature: the selenium content of food depends directly on how much selenium is in the soil where it grows. Some regions of the world have rich soils (North America), others poor ones (parts of Europe, China). Two identical carrots can have very different contents depending on where they come from.
The Brazil nut is the spectacular example: depending on the soil the tree grows in, a single nut can cover (or even exceed) your daily needs. It is the most concentrated food in the world, so much so that you should limit yourself to 1-2 a day to avoid an overdose.
The narrow window: selenium has a small safety margin. Deficiency affects the thyroid and immunity; excess (selenosis) causes hair loss, brittle nails, digestive problems and garlicky breath. It is one of the few minerals where « more » can quickly become « too much ».
The thyroid link: selenium and iodine work together for the thyroid. A selenium deficiency can worsen thyroid disorders (notably autoimmune ones such as Hashimoto’s; studies suggest a benefit from supplementation in this specific context).
Sources: Brazil nuts (the champion), fish, shellfish, offal, eggs, wholegrain cereals (depending on the soil).