Selenium
Selenium is a trace mineral that functions primarily as a component of antioxidant enzymes that protect cells from oxidative damage, and as an essential part of the system that produces and regulates thyroid hormones. Those two roles — antioxidant defense and thyroid function — make it relevant for energy levels, immune health, and metabolic regulation in ways that go beyond what most people associate with a trace mineral.
The thyroid connection is worth understanding. Selenium is required for the conversion of the inactive thyroid hormone T4 into the active form T3 that the body actually uses. Iodine gets most of the attention in thyroid nutrition discussions, but selenium is equally necessary for the thyroid to function properly. In areas of the world where selenium levels in soil are low, thyroid disorders are more prevalent — a geographic relationship that reflects how directly mineral status in the food supply affects human health.
Selenium status is heavily influenced by the selenium content of the soil where food is grown, which varies considerably by region. Brazil nuts are the most concentrated dietary source by a significant margin — a single Brazil nut can provide a full day’s requirement. That concentration is also why Brazil nuts are the one food where eating very large amounts regularly can push selenium intake toward excess. Two or three Brazil nuts a few times a week is a reasonable approach rather than eating them daily in large quantities.
Deficiency is uncommon in most developed countries with varied food supplies. Selenium toxicity from food is rare but more realistic with Brazil nuts than with any other food source — the gap between adequate and excessive intake is narrower for selenium than for most minerals.
Reference Card
Mineral type: Trace mineral Pillar: Nourish
What it does for you
- Component of antioxidant enzymes that protect cells from oxidative damage
- Essential for thyroid hormone activation — converts inactive T4 to active T3
- Supports immune function
- Involved in DNA synthesis and reproductive health
Where to get it
- Brazil nuts — by far the most concentrated source
- Tuna, halibut, sardines, shrimp, beef, turkey, eggs, cottage cheese, whole grains
Considerations
- Selenium content in food varies with soil selenium levels — regional variation is significant
- Brazil nuts are highly concentrated — two to three a few times per week provides adequate selenium without risking excess
- The gap between adequate and excessive selenium intake is narrower than for most minerals — high-dose supplementation warrants caution
- Works alongside iodine for thyroid function — both matter, not just one
Signs your intake might be low
- Fatigue and sluggishness
- Weakened immune function
- Hair loss in more significant deficiency
- Thyroid dysfunction symptoms — feeling cold, weight changes, low energy
Common myths
- Iodine is the only mineral that matters for thyroid health — selenium is equally essential for thyroid hormone activation; addressing iodine without considering selenium gives an incomplete picture
- More selenium is better for antioxidant protection — selenium has a narrow optimal range; excess over time causes toxicity, and high dose supplementation does not produce proportionally greater antioxidant benefit