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Selenium

Selenium

Best for

Improve thyroid function

Promising early signal 60–1000 mcg/day 5–52 weeks 4 meta-analyses, n=4.8k #1 of 7 studied

Selenium provides a proven benefit for sperm count and movement in men addressing fertility, while evidence across other outcomes is less consistent.

Median effective dose, most-studied outcomes

  • Bring thyroid levels into range 240 mcg 5–52 wk
  • Reduce thyroid-directed immune activity 200 mcg 8–52 wk
  • Strengthen cellular stress defenses 200 mcg 11–26 wk
Full protocol, 7 outcomes

Risk, worst reported severity

  • grade 3 or 4 adverse events severe
  • ovarian torsion requiring right oophorectomy severe
  • epirubicin major
  • lipid modifying agents major
All 18 events and interactions

What selenium actually does

Outcome Effect Evidence Dose Time Rank
Clear benefits 1
Improve sperm quality Proven benefit Sperm quality includes the number, movement, shape, and survival of sperm.
moderate 		2 meta-analyses 2 papers · n=2.2k 100–200 mcg 19–22 wk #1 /11
Modest benefits 2
Lower thyroid antibodies Likely modest benefit Thyroid antibodies can signal an immune attack on the thyroid, which makes hormones that regulate metabolism.
small 		4 meta-analyses 9 papers · n=5.1k 50–1000 mcg 8–52 wk #1 /4
Healthy thyroid function in pregnancy Proven but unnoticeable During pregnancy, the thyroid makes hormones that regulate metabolism and support fetal development.
trivial 		4 meta-analyses 9 papers · n=4.8k 60–1000 mcg 5–52 wk #3 /8
Worth watching 1
Improve thyroid function Promising early signal Thyroid hormones help regulate energy use, body temperature, heart rate, and digestion.
moderate 		4 meta-analyses 9 papers · n=4.8k 60–1000 mcg 5–52 wk #1 /7
Doesn't appear to help 1
Ease symptoms of hypothyroidism Probably doesn't help Hypothyroidism can cause fatigue, feeling cold, constipation, dry skin, and slowed thinking.
large 		1 meta-analysis 1 papers · n=679 80–200 mcg — —

Effect tiers come from native-unit MCID where available, Cohen's d otherwise.

Protocol

Dose, timing & forms

How much

For sperm quality, the better-supported protocols used 100 to 200 mcg daily for about five months, with the middle of that range representing the typical studied intake. 2 Thyroid-autoimmunity studies cover a much wider 50 to 1,000 mcg daily range, but around 200 mcg daily appears most often in the researched protocols, and the average antibody benefit is modest. 3 Do not assume more is better, since selenium has a narrow gap between an adequate intake and excessive intake. 5

When and how long

Take selenium consistently each day rather than only when you feel tired or notice thyroid symptoms. The relevant trials evaluated changes after weeks to months, not after a single dose, so pick a meal-time routine you can maintain. 23 Taking it with food is a practical choice if supplements upset your stomach. 5

Forms & standardisation

Look for the exact selenium form and amount on the label, since trials have used both selenium yeast or selenomethionine and sodium selenite. 34 Do not stack a standalone product with a multivitamin, prenatal, or fortified food without adding up the total, especially if you eat Brazil nuts regularly. 5

Studied range, by outcome

Outcome Studied range Median effective Duration Evidence
Bring thyroid levels into range 60–1,000 mcg 240 mcg 5–52 wk 9 papers 3 sig. endpoints
Reduce thyroid-directed immune activity 50–1,000 mcg 200 mcg 8–52 wk 9 papers 13 sig. endpoints
Strengthen cellular stress defenses 50–500 mcg 200 mcg 11–26 wk 7 papers 5 sig. endpoints
Support overall antioxidant capacity 200 mcg 200 mcg 8–12 wk 5 papers 5 sig. endpoints
Increase sperm concentration 100–200 mcg 150 mcg 19–22 wk 2 papers 1 sig. endpoint
Restore stable thyroid function 80–200 mcg — — 1 paper
Help sperm move forward 100–200 mcg 150 mcg 19 wk 1 paper 1 sig. endpoint

Adverse events and drug interactions

Safety events

grade 3 or 4 adverse events severe
ovarian torsion requiring right oophorectomy severe
prostate cancer diagnosis severe
Non-ST elevation myocardial infarction (serious adverse event) severe
Diagnosis of bowel cancer (serious adverse event) severe
Pulmonary embolus due to metastatic bowel cancer (serious adverse event) severe
nausea moderate
elevated PSA moderate

Drug interactions

epirubicin major increases effect
lipid modifying agents major increases toxicity
5-fluorouracil moderate increases effect
gemcitabine moderate increases effect
cisplatin moderate increases effect
paclitaxel moderate increases effect
simvastatin moderate decreases effect
roxadustat moderate decreases concentration
lithium minor decreases effect
lipid modifying agents minor unknown
Buy well

Recommended Selenium products

Real products that dose Selenium where the research does, in a clean formulation, from brands we've audited. We only list ones we'd stand behind.

Chosen for adequate Selenium dosing vs. the studied range, a clean formulation (grade A–C), and independent brand-quality audits. Not sponsored.

In context

What this is and how it works

  • Two studies (n=2,162) reported improved sperm count and movement, although the supplied evidence lacks a pooled effect size or MCID comparison. 2
  • The reviewed evidence covers 59 papers and 109 outcomes, with the clearest signal concentrated in male fertility.
  • Evidence strength varies substantially across outcomes, and fertility findings apply primarily to men with reproductive concerns.

Selenium is a trace mineral that your body needs in tiny amounts. You get it from foods such as seafood, meat, eggs, grains, and Brazil nuts, but food levels shift widely because selenium content starts in the soil. 15

Your body builds selenium into specialized proteins that keep reactive oxygen compounds from damaging cells and help switch thyroid hormone into its active form. Think of selenium as a tiny replacement part in the cell's cleanup crew and thermostat wiring: without it, those systems cannot run their usual maintenance jobs. 1

Bottom line

Selenium is most worth considering for men addressing sperm quality, ideally as part of a clinician-guided fertility plan, because that is where the human signal is clearest. 2 For Hashimoto's, expect at most a modest antibody change, not a reliable fix for thyroid symptoms or low thyroid function. 3 Avoid self-directed high-dose use, and talk with your oncology team before use if you receive cancer treatment because selenium can interact with some treatment regimens. 5

Frequently asked

Common questions

Does selenium improve sperm quality?

The clearest evidence supports sperm quality: trials in men with infertility found improvements in sperm concentration and movement after sustained supplementation, although some protocols paired selenium with another antioxidant. 2

Does selenium help Hashimoto's?

Selenium can lower thyroid antibody levels in some people with autoimmune thyroiditis, but that change tends to be small and does not reliably translate into better thyroid hormone levels or symptoms. 3

How long does selenium take to work?

Sperm-related trials ran for roughly five months, while thyroid antibody and antioxidant outcomes were usually assessed after several weeks to months. Selenium is a daily nutrient strategy, not an instant symptom reliever. 23

Can selenium replace thyroid medication?

No. In a trial involving low thyroid function, selenium did not show a meaningful improvement in thyroid status, so it is not a substitute for clinical evaluation or prescribed care. 3

Sources

Sources

  1. 1. The importance of selenium to human health (2000) ↑
  2. 2. The effect of selenium and N-acetyl-cysteine on the outcome of assisted reproductive technology in men with idiopathic infertility: a randomized placebo-controlled, double-blind study (2009) ↑
  3. 3. Selenium supplementation for autoimmune thyroiditis (2013) ↑
  4. 4. The influence of selenium supplementation on postpartum thyroid status in pregnant women with thyroid peroxidase autoantibodies (2007) ↑
  5. 5. Selenium Fact Sheet for Health Professionals

Generated August 27, 2026

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