Selenium is one of those nutrients that barely registers in most health conversations. It's not as well-known as iron or vitamin D, and GPs don't routinely test for it. But it's genuinely important, particularly if you have thyroid autoimmunity (as covered in the thyroid article), fertility concerns, or immune dysfunction.

Your body needs selenium in microgram quantities. But those micrograms support antioxidant defence, thyroid hormone conversion, immune regulation, and reproductive health. Deficiency has real clinical consequences. The catch: selenium has a narrow therapeutic window. The gap between "enough" and "too much" is smaller than for almost any other nutrient.

This article covers what selenium actually does, where the evidence is strong, where it's weak, who should test, and why "more is better" is particularly dangerous advice for this mineral.

A note before we get into it

What selenium does

Antioxidant defence. Selenium is a component of glutathione peroxidase, one of the most important antioxidant enzyme families in your body. These enzymes neutralise hydrogen peroxide and lipid peroxides, protecting cells from oxidative damage. Without adequate selenium, your antioxidant defence system is compromised at a fundamental level.

Thyroid hormone conversion. This is the function most relevant to clinical practice. The enzymes that convert T4 (inactive thyroid hormone) to T3 (active thyroid hormone) are selenoproteins. They contain selenium in their active site. The thyroid gland has the highest selenium concentration per gram of any organ in the body. Without adequate selenium, thyroid hormone metabolism is impaired, which can affect energy, metabolism, mood, and cognitive function even when TSH and T4 appear normal.

Immune regulation. Selenium influences both innate and adaptive immune responses. It modulates inflammatory pathways, supports natural killer cell function, and has a role in the immune system's ability to distinguish between "self" and "non-self," which is directly relevant to autoimmune conditions.

Reproductive health. In men, selenium is incorporated into the structural protein of sperm (selenoprotein P in the sperm tail) and protects sperm from oxidative damage. Low selenium has been associated with reduced sperm motility and morphology. In women, selenium supports ovarian function, follicular development, and may have a role in implantation success. See the pre-conception men article and the fertility article.

DNA synthesis and protection. Selenium-dependent enzymes protect DNA from oxidative damage during replication, a function relevant to both cancer prevention and healthy ageing.

Selenium and the thyroid: the evidence that actually exists

This is the area where selenium has the strongest clinical evidence.

The Hashimoto's connection. Multiple randomised controlled trials have examined selenium supplementation in patients with Hashimoto's thyroiditis. The most consistent finding: supplementation with 200 µg/day of selenomethionine reduces anti-TPO antibody titres in Hashimoto's patients, typically by 20-40% over 3-12 months of treatment.

What this means clinically. Reduced antibody levels suggest reduced autoimmune attack on the thyroid. Whether this translates to slowed disease progression, reduced need for levothyroxine dose increases, or improved clinical outcomes is less clear. The trials are mostly short-term (3-12 months), and hard clinical endpoints (thyroid failure, dose escalation) haven't been adequately studied.

What it doesn't mean. Selenium doesn't treat hypothyroidism. It doesn't replace levothyroxine. It doesn't cure Hashimoto's. It's a potential adjunct that may modulate the autoimmune process, and the evidence, while promising, is moderate rather than definitive.

The Graves' disease angle. Selenium has also been studied in Graves' disease (autoimmune hyperthyroidism). The GRASS trial found that selenium supplementation improved quality of life and eye-related outcomes in patients with mild Graves' orbitopathy. The European Thyroid Association conditionally recommends selenium for mild, recent-onset Graves' orbitopathy.

The T4 to T3 conversion angle. Because the deiodinase enzymes that convert T4 to T3 are selenoproteins, adequate selenium status supports efficient thyroid hormone activation. In selenium-deficient populations, supplementation improves thyroid hormone profiles. In selenium-replete populations, additional supplementation doesn't provide further benefit for conversion.

Selenium and fertility

Selenium's role in reproduction is increasingly recognised, though the evidence base is smaller than for thyroid.

Male fertility. Selenium is used in spermatogenesis. Selenoprotein P is a structural component of the sperm midpiece, and selenium-dependent glutathione peroxidase protects sperm from oxidative damage during maturation and transit. Several studies have found that selenium supplementation (often combined with other antioxidants) may improve sperm motility and morphology in subfertile men, though the effect size is modest and study quality is variable.

Female fertility. Selenium status has been associated with ovarian reserve, follicular fluid quality, and implantation success in IVF studies. Deficiency has been linked to recurrent miscarriage in some research. The evidence is suggestive but not yet strong enough for routine supplementation recommendations.

For couples planning conception, particularly if the male partner has suboptimal semen parameters or if selenium status is unknown, testing and supplementing if deficient is reasonable. See the pre-conception men article.

Selenium and immunity

Viral defence. Selenium deficiency has been associated with increased susceptibility to viral infections and worse outcomes from viral illness. The mechanism involves impaired production of selenoprotein-dependent immune mediators and reduced antioxidant protection in immune cells under inflammatory stress.

Autoimmune modulation. Selenium's role in redox balance and immune regulation extends to autoimmune conditions beyond thyroid, though the clinical trial evidence is strongest for thyroid autoimmunity specifically.

Immune ageing. Selenium status tends to decline with age in many populations, and this decline has been associated with immune senescence (age-related immune decline). Whether supplementing selenium in older adults improves immune function is an active area of research, with mixed results so far.

Selenium and cancer: the story that got complicated

The early promise. The Nutritional Prevention of Cancer (NPC) trial, published in 1996, found that selenium supplementation (200 µg/day) reduced the incidence of prostate, lung, and colorectal cancer in a US population. This generated enormous excitement.

The complication. The larger SELECT trial (Selenium and Vitamin E Cancer Prevention Trial), designed specifically to test selenium's effect on prostate cancer, found no benefit and actually found a slightly increased risk of type 2 diabetes in the selenium-supplemented group. The trial was stopped early.

The current understanding. The cancer-preventive effect of selenium appears to depend on baseline selenium status. In populations that are selenium-deficient, supplementation to adequate levels may reduce certain cancer risks. In populations that are already selenium-replete, additional supplementation provides no benefit and may cause harm.

Don't take selenium supplements for cancer prevention unless you're deficient. "More is better" does not apply here. The evidence supports adequacy, not excess.

Australian selenium: soil, food, and regional variation

Australia's selenium story is complicated by geography.

Soil selenium varies significantly across Australia. Some regions (parts of Queensland, Western Australia) have adequate or high soil selenium, producing selenium-rich crops and livestock. Other regions (parts of South Australia, New South Wales, Victoria) have lower soil selenium, which translates to lower selenium in locally produced food.

Most studies suggest Australian adults are generally selenium-replete. Average dietary intake meets the recommended daily intake (RDI) of 60 µg/day for women and 70 µg/day for men. However, individual variation exists, and people with restricted diets, malabsorption, or specific medical conditions may be deficient.

Dietary sources. The single most concentrated food source of selenium is the Brazil nut. One to two Brazil nuts per day provides approximately 70-140 µg of selenium, which is more than the RDI. Other sources include seafood (tuna, sardines, prawns), meat (beef, chicken, pork), eggs, wholegrains, and mushrooms.

The Brazil nut caveat. Selenium content in Brazil nuts varies enormously depending on the soil where the tree grows. A single nut can contain anywhere from 10 µg to 90+ µg. This means "two Brazil nuts a day" is a useful guideline but not a precise prescription. It also means it's possible to get too much selenium from Brazil nuts if you eat them in large quantities. Testing is the only way to know your actual status.

Who's at risk of deficiency

The narrow therapeutic window: why more is not better

This is the section that distinguishes evidence-based selenium use from supplement-industry marketing.

The recommended daily intake (RDI): 60 µg/day for women, 70 µg/day for men.

The therapeutic range for supplementation (thyroid autoimmunity): 200 µg/day, the dose used in most clinical trials.

The tolerable upper intake level: 400 µg/day. Above this, toxicity risk increases.

Selenium toxicity (selenosis). Symptoms include garlic-like breath odour, metallic taste, hair loss, brittle nails, GI disturbance, fatigue, irritability, and in severe cases, neurological effects. Chronic excessive intake can cause permanent nail and hair damage.

The gap between "therapeutic" and "toxic" is approximately 200 µg, one of the narrowest margins of any nutrient. Compare this to vitamin C (where you can consume grams before problems emerge) or vitamin D (where toxicity requires sustained doses above 10,000 IU/day). Selenium demands precision.

Don't supplement selenium without testing. Don't exceed 200 µg/day without clinical guidance. Don't combine high-dose selenium supplements with daily Brazil nut consumption without knowing your levels. And don't take selenium "just in case." Test, assess, and supplement if deficient.

How the test works

Standard blood test. Serum selenium or plasma selenium from a venous blood draw.

Fasting: Not required.

Timing: No significant circadian variation.

Preparation: Note all supplements containing selenium. Many multivitamins and thyroid-support supplements contain selenium. This isn't a reason to stop before testing (unless your clinician advises otherwise), but it helps with interpretation.

What the test reflects: Serum selenium reflects recent intake (days to weeks). It's a reasonable marker of current status for most clinical purposes. For long-term selenium stores, red blood cell selenium or selenoprotein P can be measured, though these are less commonly available.

Understanding your result

Reference range (approximate): 0.63-1.70 µmol/L (or 50-135 µg/L; units vary by lab)

Below reference range: Deficiency. Supplementation is warranted. The dose and form (selenomethionine is preferred for supplementation) should be discussed with your clinician.

Lower half of reference range: Suboptimal. May be clinically relevant if you have autoimmune thyroid disease, fertility concerns, or immune dysfunction. Supplementation to mid-range is reasonable under clinical guidance.

Mid to upper reference range: Adequate. No supplementation needed. Dietary selenium intake is sufficient.

Above reference range: Excess. Assess supplement and dietary intake (particularly Brazil nuts). Reduce intake. Retest in 3 months. Chronic excess carries toxicity risk.

Who should be testing

Tests to consider through Bloody Good

The Bloody Good Test covers 100 biomarkers including thyroid, iron, vitamin D, B12, and more. Pairing with a standalone selenium test provides complete thyroid and nutritional context, particularly useful for Hashimoto's patients and anyone investigating trace mineral status.

What to do after testing

If selenium is deficient: Supplement, typically 100-200 µg/day of selenomethionine. Retest at 3 months to confirm repletion. Address dietary sources (2 Brazil nuts daily is a simple, food-first approach, but variable content means testing confirms adequacy).

If selenium is suboptimal and you have Hashimoto's: Discuss 200 µg/day selenomethionine supplementation with your endocrinologist or GP. Monitor thyroid antibodies at 6 months. Continue levothyroxine as your primary thyroid treatment. Selenium is adjunctive, not primary.

If selenium is adequate: No supplementation needed. Dietary intake is sufficient. Don't add selenium "just in case."

If selenium is elevated: Reduce intake. Review supplements (many contain selenium you may not be aware of). Reduce Brazil nut consumption if excessive. Retest in 3 months. If persistently elevated, stop all selenium-containing supplements and reassess.

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