Selenium

non-nutrient/non-botanicalselenium atom

Take with food

What is it

Selenium is an essential trace mineral that the body incorporates into selenoproteins — enzymes critical for antioxidant defense, thyroid hormone metabolism, and immune function. The amount needed is small but matters significantly.

How it works

Selenium is incorporated into the amino acid selenocysteine, which forms the active site of about 25 selenoproteins in humans. These include glutathione peroxidases (which neutralize peroxides and protect cells from oxidative damage), thioredoxin reductases (involved in DNA synthesis and redox regulation), and the deiodinases that activate thyroid hormone by converting T4 to T3. Dietary selenium content varies enormously depending on soil selenium where food is grown. Brazil nuts are exceptionally high — a single nut can exceed the RDA. Selenium status differs dramatically between regions, with deficiency common in some parts of China and Europe.

Evidence for 5 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Correction of selenium deficiency

Grade A

Strong evidence

Supplementation reverses deficiency, which causes a cardiomyopathy called Keshan disease in low-selenium regions and increases susceptibility to certain infections.

Thyroid autoimmunity

Grade B

Good evidence

200 mcg per day reduces thyroid peroxidase antibodies in Hashimoto's thyroiditis in some trials. Whether this improves clinical outcomes is less clear.

Cognitive decline

Grade C

Moderate evidence

Some observational links between low selenium and cognitive decline; supplementation trials have not consistently shown benefit.

Cancer prevention

Grade D

Mixed evidence

Earlier optimism from observational studies and the NPC trial was not confirmed by the SELECT trial, which found no benefit and possible harm (increased type 2 diabetes risk).

Cardiovascular disease prevention

Grade D

Mixed evidence

Trials have not consistently shown selenium supplementation reduces cardiovascular events.

3 commercial forms

Selenomethionine

highly bioavailable, organic form

The form found naturally in plant and animal foods. Well absorbed (around 90 percent) and incorporated into body proteins. Common in supplements.

Selenium yeast

selenomethionine-rich, well absorbed

Yeast grown in selenium-enriched media that produces selenomethionine. Comparable to direct selenomethionine.

Sodium selenite / selenate

inorganic, well absorbed

Less expensive inorganic forms used in some supplements and in research. Bioavailability is good but slightly different metabolic fate.

Dosage

The RDA is 55 mcg per day for adults. Pregnant women need 60 mcg and breastfeeding women 70 mcg. Common supplements provide 100 to 200 mcg. The tolerable upper limit is 400 mcg per day for adults — selenium has a relatively narrow window between sufficient and toxic.

When and how to take it

Selenium absorbs well with or without food. Take consistently at the same time each day. Eating two Brazil nuts daily provides ample selenium for most people — though the wide variation in nut selenium content means routinely eating many Brazil nuts can lead to excess.

Food sources

Food	Amount	%DV
Brazil nuts, 1 oz (6-8 nuts)	544 mcg	989%
Tuna (yellowfin), 3 oz cooked	92 mcg	167%
Halibut, 3 oz cooked	47 mcg	85%
Sardines (canned), 3 oz	45 mcg	82%
Beef (chuck), 3 oz cooked	33 mcg	60%
Turkey breast, 3 oz cooked	31 mcg	56%
Egg, 1 large hard-boiled	15 mcg	27%
Cottage cheese, 1 cup	20 mcg	36%

Safety

Acute selenium toxicity from high doses causes nausea, vomiting, hair loss, brittle nails, garlic breath odor, and nerve damage. Chronic intake above 400 mcg per day can cause selenosis with similar but slower-onset symptoms. Several cases of severe toxicity have come from supplements that contained mislabeled amounts — sometimes thousands of mcg per dose. Unlike water-soluble vitamins, selenium does not have a wide safety margin. Stick close to the RDA unless your provider directs higher doses with monitoring.

Who should be cautious

Pregnant women should not exceed RDA-level doses. People with thyroid conditions should discuss selenium with their doctor — both deficiency and excess can affect thyroid function. Anyone considering long-term high-dose selenium should be aware that the SELECT trial found 200 mcg/day may have increased type 2 diabetes risk.

Interactions

Selenium may interact with anticoagulants — high doses could increase bleeding risk. Chemotherapy and radiation therapy involve oxidative damage that selenium might modify; coordinate with your oncologist. Statins and niacin combined with high-dose antioxidants including selenium may reduce HDL improvement.

Frequently asked questions

How much selenium is too much?⌄

Above 400 mcg per day from all sources can cause selenosis with hair loss, brittle nails, and nerve symptoms. Stick to 200 mcg/day or less unless directed otherwise.

Are Brazil nuts a reliable selenium source?⌄

They are very rich but extremely variable — a single nut can contain anywhere from 50 to 300+ mcg. Eating 1 to 2 Brazil nuts per day is usually safe and effective; routine consumption of many can cause excess.

Should I take selenium for thyroid issues?⌄

200 mcg/day reduces thyroid antibodies in Hashimoto's in some trials, but whether this improves outcomes is unclear. Discuss with your endocrinologist.

Does selenium prevent cancer?⌄

Despite earlier hopes, the large SELECT trial found no benefit and possible harm (increased diabetes risk). Routine supplementation for cancer prevention is not recommended.

What form of selenium is best?⌄

Selenomethionine or selenium yeast are well absorbed and incorporated into body proteins. Inorganic forms also work.

Track Selenium with Pilora

Set up dose reminders, check interactions, and join the community in the Pilora iPhone app.

Coming to App Store

Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.