Selenium
Useful mainly for people with confirmed deficiency or Hashimoto's thyroiditis with elevated antibodies.
Quick decision guide
May help most
People with confirmed deficiency or Hashimoto's thyroiditis with elevated antibodies
Common dosing range
55-200 mcg/day
When to expect effects
Weeks to months
Watch out for
Narrow margin between adequate and toxic - do not exceed 400 mcg/day from all sources
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.
Is it worth it for you?
Use this as a quick fit check, not a diagnosis.
Worth considering if…
Probably skip if…
Evidence at a glance
| Goal | Effect | Best fit | Time |
|---|---|---|---|
selenium deficiency correction Strong Evidence | Definitive; resolves deficiency at recommended doses | People with low dietary intake or confirmed deficiency | Weeks |
thyroid autoimmunity (Hashimoto's thyroiditis) Good Evidence | Modest reduction in TPO antibody titers in most RCTs | People with Hashimoto's thyroiditis and elevated anti-TPO or anti-TG antibodies | 3-6 months |
cancer prevention Mixed Evidence | No benefit demonstrated; possible harm at supplemental doses | No population currently identified as benefiting | Years |
selenium deficiency correction
- Effect
- Definitive; resolves deficiency at recommended doses
- Best fit
- People with low dietary intake or confirmed deficiency
- Time
- Weeks
thyroid autoimmunity (Hashimoto's thyroiditis)
- Effect
- Modest reduction in TPO antibody titers in most RCTs
- Best fit
- People with Hashimoto's thyroiditis and elevated anti-TPO or anti-TG antibodies
- Time
- 3-6 months
cancer prevention
- Effect
- No benefit demonstrated; possible harm at supplemental doses
- Best fit
- No population currently identified as benefiting
- Time
- Years
Evidence for 3 uses
AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.
selenium deficiency correction
Corrects deficiencySelenium is an essential trace element incorporated into approximately 25 selenoproteins critical for antioxidant defense (glutathione peroxidases), thyroid hormone conversion (deiodinases), and immune regulation. Severe deficiency causes Keshan disease (endemic cardiomyopathy) and Kashin-Beck disease (osteoarthropathy). Supplementation at RDA-level doses (55 mcg/day) reliably restores selenoprotein activity in deficient individuals.
Bottom line: Supplementation definitively corrects deficiency and the associated functional consequences when dietary intake is inadequate.
thyroid autoimmunity (Hashimoto's thyroiditis)
Biomarker supportMultiple small-to-moderate RCTs and several meta-analyses show that 200 mcg/day of selenomethionine reduces circulating anti-TPO antibody titers in Hashimoto's thyroiditis. However, whether this antibody reduction translates to clinically meaningful hard endpoints is less clear: the larger CATALYST trial (n=472, 18 months) did not find improvement in thyroid function, quality of life, or levothyroxine dose requirements.
Bottom line: Selenium reduces Hashimoto's antibody biomarkers in blood; clinical benefit on hard outcomes is not yet consistently demonstrated.
Evidence is mixed
Multiple meta-analyses confirm TPO antibody reduction, but the 2021 CATALYST RCT (the largest to date) showed no benefit on thyroid function, quality of life, or clinical outcomes over 18 months.
cancer prevention
Supplement benefitThe SELECT trial (n=35,533 men, 2001-2011) found 200 mcg/day selenium did not reduce prostate cancer incidence and was associated with a non-significant increased risk of type 2 diabetes. Earlier observational studies and the Nutritional Prevention of Cancer trial had suggested benefit, but these findings were not replicated in the large well-powered RCT. Current evidence does not support selenium supplementation for cancer prevention.
Bottom line: Selenium does not prevent cancer; large RCT evidence is null and raises a possible diabetes risk signal - do not supplement for cancer prevention.
Evidence is mixed
Early NPC trial data and observational studies suggested cancer protection; the large SELECT RCT definitively showed no benefit and raised a diabetes signal, reversing the earlier optimism.
How it works
How to take it
What to track
3 commercial forms
Compare the main delivery options and what they’re best suited for.
Selenomethionine
The form found naturally in plant and animal foods. Well absorbed (around 90 percent) and incorporated into body proteins. Common in supplements.
highly bioavailable, organic form
Selenium yeast
Yeast grown in selenium-enriched media that produces selenomethionine. Comparable to direct selenomethionine.
selenomethionine-rich, well absorbed
Sodium selenite / selenate
Less expensive inorganic forms used in some supplements and in research. Bioavailability is good but slightly different metabolic fate.
inorganic, well absorbed
Safety
Know the common side effects, key cautions, and who should avoid it.
Common side effects
Serious risks
Selenosis above 400 mcg/day: hair loss, brittle nails, peripheral neuropathy, garlic breath odor
Mislabeled supplements have caused acute toxicity at thousands of mcg per dose
Possible increased type 2 diabetes risk at 200 mcg/day (SELECT trial signal)
Who should avoid it
- People already meeting selenium needs through diet - no additional benefit and adds toxicity risk
- People with diabetes risk factors at doses above 55 mcg/day (discuss with physician)
- People taking supplements with unknown selenium content alongside high-selenium foods
Pregnancy & breastfeeding
Pregnant women need 60 mcg/day; do not exceed this without medical supervision - selenium has a narrow margin between adequate and toxic.
Interactions
Antioxidant activity may theoretically counteract oxidative mechanisms of cancer treatment; coordinate with oncologist
High doses may modestly increase bleeding risk
High-dose antioxidant combinations including selenium may blunt the HDL-raising effect of niacin
Documented interactions
Evidence-graded pair pages with sources, dosing notes, and timing guidance — a complement to the narrative section above.
Beneficial pairs (3)
+ iodine
synergyIodine is the raw material the thyroid uses to build the hormones T4 and T3, but selenium is required to make the deiodinase enzymes that convert inactive T4 into active T3 in peripheral tissues. Selenium also powers glutathione peroxidase, which protects thyroid cells from the oxidative stress generated during iodine handling. The two minerals work as a pair: each is far less useful without the other.
+ nac
synergyNAC supplies cysteine, the rate-limiting building block for glutathione synthesis, while selenium is the cofactor built into the glutathione peroxidase enzymes that use glutathione to neutralize peroxides. The two nutrients support the same antioxidant pathway, so on a mechanistic level each helps the other work. Combined clinical benefit beyond that shared pathway is not well demonstrated, and the pairing is low-risk.
+ vitamin e
synergyVitamin E and selenium are complementary antioxidants. Selenium is the cofactor for glutathione peroxidase, which clears lipid peroxides and spares vitamin E, while vitamin E intercepts free radicals in membranes and reduces the demand on the selenium-dependent enzyme. The partnership is well established in animal and mechanistic studies; clinical benefit of the combination in people is more limited.
Protocols featuring Selenium
Evidence-backed routines where Selenium plays a role.
Thyroid Support — Hashimoto's
thyroid
Hashimoto''s thyroiditis is the most common cause of hypothyroidism in iodine-replete countries — autoimmune destruction of thyroid tissue driving elevated TPO antibodies and eventual hypothyroid state. Treatment of confirmed hypothyroidism is levothyroxine; supplements DO NOT replace thyroid hormone replacement. They CAN reduce TPO antibody levels, support thyroid function in early/subclinical Hashimoto''s, and address common cofactor deficiencies that worsen disease progression. The strongest evidence in the supplement category is for selenium (Grade A in recent meta-analyses for TPO antibody reduction), vitamin D3 (Grade B), and the combination of myo-inositol + selenium (Grade B). If you have a confirmed Hashimoto''s diagnosis, this stack complements your endocrinologist''s management, doesn''t replace it. If you suspect Hashimoto''s, get TSH, free T4, free T3, TPO antibodies, and thyroglobulin antibodies before starting.
Men's Fertility / Sperm Health
maternal
Up to 50% of infertility cases involve a male factor — yet most fertility workups focus disproportionately on the female partner. The 90 days before conception matter for men too: spermatogenesis takes 72-74 days, so the nutritional and lifestyle environment during that window directly affects sperm count, motility, morphology, and DNA fragmentation. The supplement category here has unusually clear evidence: CoQ10 (ubiquinol) for motility and count, zinc for foundational spermatogenesis, L-carnitine for motility specifically, selenium for sperm glutathione peroxidase activity, and ashwagandha for testosterone + sperm parameters. Effect sizes are real and replicated in multiple trials. If you''ve been trying to conceive for 12+ months (or 6+ months if your partner is 35+) without success, get a semen analysis — it''s cheap, fast, and informative. Don''t default to assuming the issue is female-only.
Thyroid Foundation (Hypo)
thyroid
Hypothyroidism — outside of autoimmune Hashimoto''s — is most commonly due to iodine deficiency in some populations, selenium deficiency, or post-medical causes (radiation, surgery, medication-induced). In iodine-replete countries, autoimmune Hashimoto''s accounts for the majority of cases (see the Hashimoto''s protocol). This protocol is for non-autoimmune hypothyroidism or subclinical hypothyroidism without elevated TPO antibodies — selenium, low-dose iodine (only if deficiency is documented), tyrosine (precursor to thyroid hormones), and B12 for the fatigue often accompanying hypothyroidism. If you have confirmed Hashimoto''s (positive TPO antibodies), use that protocol instead — iodine supplementation is potentially harmful in autoimmune thyroid disease. Treatment of confirmed hypothyroidism is levothyroxine. Supplements do not replace thyroid hormone replacement. They support endogenous function and address common cofactor deficiencies.
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% |
Brazil nuts, 1 oz (6-8 nuts)
- Amount
- 544 mcg
- %DV
- 989%
Tuna (yellowfin), 3 oz cooked
- Amount
- 92 mcg
- %DV
- 167%
Halibut, 3 oz cooked
- Amount
- 47 mcg
- %DV
- 85%
Sardines (canned), 3 oz
- Amount
- 45 mcg
- %DV
- 82%
Beef (chuck), 3 oz cooked
- Amount
- 33 mcg
- %DV
- 60%
Turkey breast, 3 oz cooked
- Amount
- 31 mcg
- %DV
- 56%
Egg, 1 large hard-boiled
- Amount
- 15 mcg
- %DV
- 27%
Cottage cheese, 1 cup
- Amount
- 20 mcg
- %DV
- 36%
Choosing a product
What to look for on the label — and what to be skeptical of.
Look for…
Be skeptical of…
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.
References by claim
selenium deficiency correction
thyroid autoimmunity (Hashimoto's thyroiditis)
Safety
NIH Office of Dietary Supplements — Selenium — NIH ODS link
Track Selenium with Pilora
Set up dose reminders, check interactions, and join the community in the Pilora iPhone app.
Coming to App StoreDisclaimer: 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.