Selenium
For informational purposes only — not medical advice. Always consult a qualified healthcare provider before making changes to your health regimen. Full disclaimer →
- Selenium has a U-shaped dose-response curve — more is not better. This distinguishes selenium from most other micronutrients. Supplementing selenium when you are already sufficient does not provide additional benefit and may cause harm. Testing before supplementing is particularly important with selenium.
- Selenium is essential for thyroid function. The enzyme that converts the prohormone T4 to the active thyroid hormone T3 — iodothyronine deiodinase — is a selenoprotein. Selenium deficiency impairs T4-to-T3 conversion and can cause functional hypothyroidism despite normal TSH and T4 levels. Selenium deficiency also impairs the thyroid peroxidase antioxidant defense system, contributing to autoimmune thyroid disease (Hashimoto's).
- Selenium is a central component of the glutathione system. Glutathione peroxidases — selenoproteins that reduce hydrogen peroxide and lipid peroxides — are among the most important cellular antioxidant enzymes. Selenium deficiency reduces glutathione peroxidase activity, increasing cellular oxidative stress and contributing to the same biological aging pathways that low glutathione status drives.
- Selenium geography matters enormously. Soil selenium content in the US and Canada is generally adequate to sufficient, so dietary deficiency is less common than in many parts of Europe, China, and New Zealand where soils are selenium-poor. Brazil nuts from selenium-rich soils in the Amazon basin are the most selenium-dense food available — a single nut can provide the full RDA.
- Selenium and iodine work synergistically for thyroid health. Both minerals are required for optimal thyroid function. Iodine deficiency combined with selenium deficiency is more damaging to thyroid function than either alone — a critical consideration for people in iodine-deficient areas or those following low-iodine diets.
The Mineral Where More Is Not Better
Most discussions of nutrient deficiency follow the same basic logic: deficiency is bad, so more is better, up to a limit. Selenium is a genuine exception to this pattern. The relationship between selenium status and health outcomes follows a U-shaped curve — deficiency is harmful, but excess is also harmful, and the optimal range sits in a window that is narrower than most micronutrients.
This distinction has real clinical consequences. The SELECT trial (Selenium and Vitamin E Cancer Prevention Trial) — which enrolled 35,533 men and supplemented them with 200 mcg/day of selenomethionine — found no cancer prevention benefit and a statistically significant increase in type 2 diabetes risk in the selenium arm. Subsequent analysis revealed that the men enrolled were predominantly already selenium-sufficient at baseline. They had nothing to gain from supplementation and potentially something to lose. 1
This trial is often cited as evidence against selenium supplementation, but the more precise conclusion is that selenium supplementation in selenium-sufficient individuals is not beneficial and may be harmful. In selenium-deficient individuals, the evidence for benefit — particularly for thyroid function, immune competence, and antioxidant capacity — is strong. The implication is straightforward: test before supplementing.
Selenoproteins and Their Roles in Aging
The biological activity of selenium is mediated almost entirely through selenoproteins — proteins that contain the selenium-incorporating amino acid selenocysteine. Twenty-five human selenoproteins have been characterized, and their functions span several of the most aging-relevant biological systems:
Glutathione peroxidases (GPx1–4): These enzymes catalyze the reduction of hydrogen peroxide and lipid peroxides using glutathione as the electron donor. They are the primary cellular defense against lipid peroxidation and mitochondrial oxidative damage. GPx4 specifically protects against phospholipid peroxidation — the form of oxidative damage implicated in ferroptosis, a form of cell death increasingly recognized as relevant to neurodegeneration and other aging-related pathologies.
Thioredoxin reductases (TrxR): These enzymes maintain the thioredoxin system — a parallel antioxidant and redox signaling pathway to the glutathione system. TrxR is involved in DNA synthesis, cell proliferation, and the regulation of transcription factors including NF-κB and p53.
Iodothyronine deiodinases (DIO1–3): These enzymes control the activation and inactivation of thyroid hormones — one of the central regulators of metabolic rate, body composition, energy, and virtually every aspect of physiological function discussed elsewhere on this site.
| Level | Range | Interpretation | Notes |
|---|---|---|---|
| Optimal | 90–120 ng/mL | Full selenoprotein activity | Target for longevity; selenoproteins maximally expressed |
| Borderline | 70–90 ng/mL | Suboptimal — consider supplementation | Selenoprotein activity may be partially impaired |
| Deficient | < 70 ng/mL | Selenium deficiency | Supplement; check thyroid function; assess diet |
| Elevated | 120–150 ng/mL | Above optimal — do not supplement further | Review diet and supplement use |
| Excess | > 150 ng/mL | Potential adverse effects | Reduce intake; assess for selenosis symptoms |
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Analyze My Biomarkers →Selenium, Autoimmune Thyroid Disease, and Hashimoto's
One of the most clinically actionable applications of selenium knowledge is in autoimmune thyroid disease. Hashimoto's thyroiditis — in which immune cells attack the thyroid — is the most common autoimmune condition in developed countries, affecting an estimated 1–5% of the population. Multiple randomized controlled trials have found that selenium supplementation (typically 200 mcg/day of selenomethionine) significantly reduces thyroid peroxidase antibody (TPO-Ab) titers in patients with autoimmune thyroiditis. 2
The proposed mechanism involves selenium's role in protecting thyroid cells from oxidative damage. Thyroid hormone synthesis generates hydrogen peroxide as a byproduct — and selenium-dependent glutathione peroxidases are the primary defense against the resulting oxidative stress within the thyroid gland. Selenium deficiency leaves thyroid cells vulnerable to oxidative damage, which may increase the presentation of modified thyroid antigens to the immune system and perpetuate the autoimmune cycle.
This evidence has led many integrative and functional medicine practitioners to routinely check selenium levels in patients with Hashimoto's and autoimmune thyroid disease, and to supplement when levels are suboptimal. Mainstream endocrinology has been more cautious, but the evidence for benefit in selenium-deficient patients with active Hashimoto's is reasonably strong.
Sources
| Range Type | Value (ng/mL) | Notes |
|---|---|---|
| Standard Clinical Range | Serum: 70–150 ng/mL | Designed to identify disease risk — not longevity optimisation. |
| Longevity-Optimal Target | Serum: 90–120 ng/mL |
Associated with reduced all-cause mortality and extended healthspan.
The U-shaped relationship between selenium and health outcomes is well-documented. Serum selenium below 85 ng/mL is associated with increased cancer risk, impaired immune function, and reduced thyroid hormone activation. Levels above 130–140 ng/mL begin to show diminishing or adverse effects in some studies. The OPTIMAL zone is sufficiency — generally 90–120 ng/mL — not maximization. This is a critical distinction from most nutrients.
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Why is selenium important for thyroid health?
The thyroid gland has the highest selenium concentration per gram of any organ in the body — a reflection of how selenium-dependent thyroid function is. Three selenoprotein families are critical to thyroid biology. Iodothyronine deiodinases (types 1, 2, and 3) catalyze the conversion of the prohormone thyroxine (T4) to the active thyroid hormone triiodothyronine (T3) — and also the inactivation of thyroid hormones when levels are too high. Without adequate selenium, this conversion is impaired and T3 production falls even when T4 is normal. Glutathione peroxidases protect the thyroid gland itself from oxidative damage during thyroid hormone synthesis (which generates hydrogen peroxide as a byproduct). Thioredoxin reductases also contribute to the thyroid antioxidant defense. Because selenium deficiency impairs the antioxidant protection of thyroid cells, it is associated with higher rates of thyroid autoimmunity — Hashimoto's thyroiditis patients consistently have lower selenium levels than controls, and selenium supplementation reduces thyroid antibody titers in several randomized controlled trials.
How much selenium is in a Brazil nut?
Brazil nuts are extraordinarily selenium-rich — but the selenium content varies dramatically depending on where they were grown, because it reflects soil selenium concentration. Brazil nuts grown in the selenium-rich soils of the Amazon basin (particularly in Brazil and Bolivia) can contain 70–90 mcg of selenium per nut, while those from selenium-poor soils may contain 10–20 mcg per nut. This variability makes Brazil nuts an unreliable sole source of selenium for people trying to optimize levels precisely. As a general guideline, one to two Brazil nuts per day from selenium-rich sources provides approximately 70–180 mcg — within the range needed for selenoprotein optimization. The RDA for selenium is 55 mcg/day for adults; the tolerable upper intake level is 400 mcg/day. A single Brazil nut from a high-selenium source can provide the full RDA.
What are the symptoms of selenium deficiency?
Mild to moderate selenium deficiency is often asymptomatic — there are no reliable clinical signs that clearly distinguish selenium deficiency from other conditions, which is why testing is necessary for accurate assessment. More severe deficiency produces fatigue and muscle weakness (from impaired mitochondrial antioxidant protection), thyroid dysfunction (particularly reduced T3 with normal T4 and TSH), immune suppression, and impaired fertility. Keshan disease — a cardiomyopathy caused by severe selenium deficiency — was first described in selenium-poor regions of China and is the most clinically severe consequence of selenium deficiency; it does not occur in populations with even moderate selenium intake. In adults in developed countries, selenium deficiency manifests primarily as thyroid dysfunction, impaired immune response, and elevated oxidative stress markers.
What are the symptoms of selenium excess (selenosis)?
Selenosis — selenium toxicity — typically results from excessive supplementation rather than dietary intake. Early signs include a garlic-like breath odor (from dimethylselenide exhalation), brittle nails that crack and fall off, hair loss, nausea, fatigue, and irritability. More severe toxicity produces peripheral neuropathy, skin lesions, and in extreme cases cardiovascular and respiratory failure. The SELECT trial — which used selenium supplementation at 200 mcg/day in men with already-adequate selenium status — found an increased risk of type 2 diabetes in the selenium arm, providing important evidence that supplementing above sufficiency is not benign. The tolerable upper intake level of 400 mcg/day is a conservative limit; toxicity symptoms typically appear with consistent intake above 600–900 mcg/day from supplements.
Should I supplement selenium or just eat Brazil nuts?
The answer depends on your current selenium status, which is why testing is the first step. If you are selenium-sufficient (90–120 ng/mL serum), you do not need to supplement and doing so carries the risk of overshooting into the adverse range. If you are deficient or borderline (below 85 ng/mL), the choice between Brazil nuts and supplements is primarily one of preference and reliability. One to two Brazil nuts per day from high-selenium sources is a dietary approach that many people prefer. Supplemental selenium at 100–200 mcg/day of selenomethionine (the most bioavailable organic form) is a reliable alternative. Sodium selenite (the inorganic form in cheaper supplements) is less bioavailable and more prone to oxidative interference. Retest after 3–6 months to confirm levels have reached the target range and are not overshooting.