Selenium
SupplementThe small divergence between the medical score [s5, s6, s7, s8] and the community score [c1, c2] is explained by the fact that both sources show limited and partly contradictory efficacy data. The community selectively reports positive experiences among individuals with Hashimoto's [c1, c2], while clinical research highlights the low quality of evidence [s6, s7].
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TL;DR
Selenium is an essential trace element with solid evidence for its role in thyroid function and antioxidant defense — but the therapeutic window is dangerously narrow. At 200 µg/day as selenomethionine, studies show meaningful reductions in TPO antibodies in Hashimoto's patients; however, supplementing without a deficiency offers no benefit and real toxicity risk. Overdosing (>300 µg/day from all sources) paradoxically causes the very symptoms many users are trying to prevent: hair loss and fatigue. Always test selenium status before supplementing — dosing blind is not advisable here.
Description
Essential trace element with antioxidant activity; important for thyroid function, immune system, and male fertility [s1, s2].
Selenium is an essential trace element that the body cannot synthesize itself and must be obtained through diet or dietary supplements [s1]. In the human body, selenium is incorporated primarily as selenocysteine in selenoproteins, which include glutathione peroxidases (GPx), thioredoxin reductases, and iodothyronine deiodinases [s1, s2]. Europe – and the DACH region in particular – is considered a selenium-poor region. Soils in Central Europe contain little selenium, which is why plant-based foods from this region have lower selenium content than those from selenium-rich regions such as North America [s3]. The average selenium intake in Germany is below the estimated value for adequate intake [s11]. Seleномethionine is considered the most bioavailable organic form and is absorbed via amino acid transporters in the small intestine [s4]. Inorganic forms (sodium selenite, sodium selenate) are absorbed via different transporters and have slightly lower bioavailability [s4]. The most clinically well-established role of selenium is in Hashimoto's thyroiditis, where supplementation can reduce thyroid peroxidase antibodies (TPO-Ab) [s5, s6]. However, the quality of evidence for most clinical endpoints is low to very low [s6, s7]. The SELECT trial (n=35,533) showed no protective effect of selenium (200 µg/day as selenomethionine) against prostate cancer in selenium-sufficient men [s8]. For male fertility, there is evidence of improvements in sperm parameters, but no consistent RCT data on birth rates [s9].
Legal Status (DE)
In Germany and the EU, selenium is marketable as an over-the-counter dietary supplement. Approved selenium compounds under EU Directive 2002/46/EC include sodium selenite, sodium selenate, and selenomethionine. The BfR recommends a maximum amount of 40 µg selenium per daily dose in dietary supplements [s10, s11].
Mechanism of Action
Selenium exerts its effects primarily as a component of selenoproteins [s1, s2]: 1. Antioxidant protection: Glutathione peroxidases (GPx1–GPx4) catalyze the reduction of hydrogen peroxide and organic hydroperoxides, thereby minimizing oxidative stress and cellular damage [s1, s2]. GPx4 specifically protects phospholipids in cell membranes, which is particularly relevant for sperm and neuronal cells [s9]. 2. Thyroid hormone metabolism: Iodothyronine deiodinases (DIO1, DIO2, DIO3) are selenoproteins that catalyze the conversion of thyroxine (T4) to the biologically active triiodothyronine (T3). In selenium deficiency, this conversion is impaired, which can lead to functional hypothyroidism [s2, s3]. 3. Immunomodulation: Selenoproteins regulate the function of macrophages and neutrophils. GPx protects phagocytic cells from oxidative damage during pathogen elimination [s1]. Low selenium levels are associated with reduced T-cell proliferation and NK-cell activity [s1]. 4. Reduction of thyroid autoimmunity: In Hashimoto's thyroiditis, selenium is thought to promote selenoprotein P expression in the thyroid gland, reduce oxidative stress in thyroid tissue, and thereby attenuate the inflammatory response and TPO antibody production [s5, s6]. 5. Male fertility: Selenoprotein P and GPx4 (also known as phospholipid hydroperoxide glutathione peroxidase) are essential for the structural integrity of the sperm midpiece and sperm motility [s9].
Dosing
Selenmangel-Ausgleich / allgemeine Gesundheit
- Dose
- 55–70 µg selenium daily
- Frequency
- 1× täglich
- Route
- oral
- Duration
- fortlaufend
- Timing
- With a meal
- With food
- empfohlen
Hashimoto-Thyreoiditis (adjuvant)
- Dose
- 200 µg selenium daily (as selenomethionine or sodium selenite)
- Frequency
- 1× täglich
- Route
- oral
- Duration
- 3–6 Monate, dann Reevaluation
- Timing
- With a meal
- With food
- empfohlen
Männliche Fertilität (Kombination)
- Dose
- 200 µg selenium + vitamin E (400 IU) daily
- Frequency
- 1× täglich
- Route
- oral
- Duration
- 3–6 Monate
- Timing
- With a meal
- With food
- empfohlen
EFSA tolerable upper intake level (UL): 255 µg/day for adults from all sources (food + supplements) [s12]. BfR recommends a maximum of 40 µg selenium per daily dose from dietary supplements [s11]. NIH/FDA: 400 µg/day as the absolute upper limit from all sources [s10].
Before initiating selenium supplementation, individual selenium status (serum selenium or selenoprotein P) should be determined, as supplementation in individuals with adequate supply provides no benefit and carries an increased risk of toxicity [s8, s11]. Selenomethionine has higher bioavailability than inorganic forms [s4].
Side Effects
| Side Effect | Frequency | Severity |
|---|---|---|
| Haarausfall (diffus) Chronic selenium oversupply (selenosis) causes diffuse hair loss. Cases have been reported from the community even at doses of 200 µg/day [s10, c1, c3]. | gelegentlich | moderat |
| Nagelveränderungen (Brüchigkeit, Verlust) Classic sign of chronic selenosis; with persistently elevated intake above the UL [s10, s12]. | gelegentlich | moderat |
| Knoblauchgeruch im Atem (Dimethylselenid) Dimethylselenide is exhaled when selenium is in excess and is an early sign of oversupply [s10]. | gelegentlich | leicht |
| Übelkeit, Erbrechen, Durchfall Gastrointestinal complaints at higher doses or acute overdose [s10]. | gelegentlich | leicht |
| Müdigkeit, Reizbarkeit, Nervenschäden Chronic oversupply (selenosis) can produce neurological symptoms such as fatigue, irritability, and peripheral neuropathy [s10]. | selten | moderat |
| Metallischer Geschmack im Mund Common early symptom of elevated selenium exposure [s10]. | gelegentlich | leicht |
| Schlafstörungen Individual users in German forums (ht-mb.de) report sleep problems with selenium supplementation [c3]. | selten | leicht |
Contraindications
Any additional selenium intake worsens toxicity. Determine serum selenium or selenoprotein P before supplementation [s10, s12].
Impaired renal selenium excretion increases the risk of accumulation and selenosis [s10].
Increased selenium intake without confirmed deficiency carries teratogenic risk. Only intake at the level of daily requirements (60 µg/day) is recommended [s10, s11].
Selenium may affect the efficacy of chemotherapeutic agents; consultation with oncology team required [s8].
Interactions
Synergistic
Selenium, as an essential cofactor for selenoproteins, participates in antioxidant defense and may support the biosynthesis and bioavailability of coenzyme Q10. Both substances act synergistically in protecting mitochondrial membranes against oxidative stress and in maintaining cellular energy production.
Iodine and selenium act synergistically to support healthy thyroid function. Selenium is required for the conversion of T4 to the more active T3 and protects the thyroid gland from oxidative stress generated during iodine processing.
The combination of coenzyme Q10 (200 mg/day) and selenium (200 µg/day) significantly reduced cardiovascular mortality risk in older adults in the KiSel-10 study. The protective effect was still detectable 12 years after the intervention.
Alpha-lipoic acid and selenium act synergistically as antioxidants and can more effectively reduce oxidative stress in combination. Alpha-lipoic acid also promotes the recycling of other antioxidants, including selenium-dependent systems.
Caution
Zinc and selenium may compete for absorption via shared transport proteins. At very high zinc doses, selenium uptake may be impaired; a staggered intake interval is recommended.
Studies
Tier A — High Evidence
Outcome: TPO-Ab reduction in LT4-treated patients: SMD = −0.53 (3 months), SMD = −1.95 (6 months); Tg-Ab not significantly reduced [s6].
Effect Size: SMD = −0.53 to −1.95; very low quality of evidence (GRADE)
Outcome: Selenium + vitamin E significantly improves sperm motility and normal morphology; no consistent effect on birth rate [s9].
Effect Size: Significant improvement in sperm parameters (meta-analysis of 8 studies)
Outcome: Reduction of TPO antibodies (SMD = −0.46 at 3 months; SMD = −0.80 at 6 months, p < 0.05) in Hashimoto's patients [s5].
Effect Size: SMD = −0.46 (3 months), −0.80 (6 months); low to very low quality of evidence (GRADE)
Outcome: Selenium (200 µg/day as selenomethionine) showed no preventive effect on prostate cancer or other cancers in selenium-sufficient men [s8].
Effect Size: RR ~1.0 (no significant effect); study terminated early
Tier B — Moderate Evidence
Outcome: Insufficient evidence for the use of selenium in the treatment of Hashimoto's thyroiditis [s7].
Effect Size: No significant recommendation possible (low evidence)
Outcome: Selenium status associated with all-cause mortality in observational studies; no reduction in mortality through supplementation in RCT meta-analyses [s13].
Effect Size: Meta-analysis of RCTs: no significant effect on mortality
Tier C — Low Evidence
Outcome: Selenomethionine and Se-methylselenocysteine showed the highest membrane permeability in Caco-2 cells; selenomethionine highest bioavailability in vivo [s4].
Effect Size: Selenomethionine > inorganic forms in bioavailability
Community Evidence
Top reported benefits
- Improvement of hair loss and nail quality (in individuals with Hashimoto's)
- Subjective improvement in well-being in thyroid conditions
- Reduction of TPO antibodies (reported following laboratory monitoring)
- Improvement in energy levels and fatigue symptoms
Top reported issues
- Hair loss with overdose (>200 µg/day)
- Sleep disturbances with evening administration
- Fatigue and irritability at excessively high doses
- No noticeable effect in many users without confirmed deficiency
Several community reports warn of selenium's narrow therapeutic window [c1, c3]. The risk of overdose is frequently underestimated, particularly when selenium-containing multivitamin supplements are already being taken. Stiftung Warentest rates selenium dietary supplements as "not very helpful – potentially harmful" [c5]. Brazil nuts as a selenium source are considered difficult to dose accurately within the community [c1].
Scientific Sources
- Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials
Huwiler VV, Maissen-Abgottspon S, Stanga Z, Mühlebach S, Trepp R, Bally L, Bano A (2024). ThyroidAPMID:38243784DOI - Effects and Impact of Selenium on Human Health, A Review
Zhang Y, Roh YJ, Han SJ, et al. (2025). International Journal of Molecular SciencesBDOI - Selenium – Health Professional Fact Sheet
National Institutes of Health, Office of Dietary Supplements (2024). NIH Office of Dietary SupplementsBLink - Aktualisierung (2023) Höchstmengenvorschläge für Selen in Lebensmitteln inklusive Nahrungsergänzungsmitteln – Stellungnahme 010/2024
Bundesinstitut für Risikobewertung (BfR) (2024). BfRADOI - Scientific opinion on the tolerable upper intake level for selenium
EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) (2023). EFSA JournalAPMID:36698500DOI - Associations of selenium status with all-cause and cause-specific mortality: a systematic review and meta-analysis of cohort studies
Multiple authors (2025). ScienceDirectALink - Cardiovascular mortality and N-terminal-proBNP reduced after combined selenium and coenzyme Q10 supplementation: a 5-year prospective randomized double-blind placebo-controlled trial among elderly Swedish citizens
["Alehagen, Urban","Johansson, Peter","Björnstedt, Mikael","Rosén, Anders","Dahlström, Ulf"] (2013). International Journal of CardiologyCPMID:22626835DOI - Still reduced cardiovascular mortality 12 years after supplementation with selenium and coenzyme Q10 for four years: A validation of previous 10-year follow-up results of a prospective randomized double-blind placebo-controlled trial in elderly
["Alehagen, Urban","Aaseth, Jan","Alexander, Jan","Johansson, Peter"] (2018). PLOS ONECPMID:29641571DOI - Selenium – The Nutrition Source
Harvard T.H. Chan School of Public Health (2023). Harvard T.H. Chan School of Public Health – The Nutrition SourceBLink - Selenium Deficiency Due to Diet, Pregnancy, Severe Illness, or COVID-19—A Preventable Trigger for Autoimmune Disease
Watad A, Bragazzi NL, McGonagle D, et al. (2021). International Journal of Molecular SciencesBDOI - Bioavailability Comparison of Nine Bioselenocompounds In Vitro and In Vivo
Wang MQ, Du YJ, Wang C, et al. (2017). Biological Trace Element Research / PMCCLink - Clinical efficacy of selenium supplementation in patients with Hashimoto thyroiditis: a systematic review and meta-analysis
Huang Y, Zhao W, Chen Z, et al. (2025). Medicine (Wolters Kluwer)ADOI - The Effects of Selenium Supplementation in the Treatment of Autoimmune Thyroiditis: An Overview of Systematic Reviews
Ventura M, Melo M, Carrilho F, et al. (2023). NutrientsADOI - Nahrungsergänzung mit Selen bei Hashimoto-Schilddrüsenentzündung (Cochrane Review)
Cochrane Collaboration (2013). Cochrane Database of Systematic ReviewsALink - Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT)
Lippman SM, Klein EA, Goodman PJ, et al. (2009). JAMAAPMID:19066370DOI - Therapeutic efficacy of antioxidant supplementation containing selenium and vitamin E on male infertility: A systematic review and meta-analysis
Multiple authors (2026). ScienceDirect (Reproductive Medicine)ALink
Community Sources
Storage
Unopened
Store dry and cool at room temperature (15–25 °C), protected from light and moisture.
Opened
Keep container tightly closed; refrigeration is not required for capsule and tablet forms.
Notes
Selenium compounds are generally stable. Selenium yeast products should be protected from moisture, as increased moisture exposure can impair bioavailability.