Supplement · graded by use, two uses
Selenium (Selenomethionine)
Autoimmune thyroiditis (TPO antibodies).
Grade C: human trials exist, but they are small, mixed or not yet repeated. This is not a finding that it does not work. How to read a grade →
Selenium is a mineral the thyroid gland needs. In people with an autoimmune thyroid disease, selenium lowered the levels of antibodies that attack the thyroid. Several studies agree on that. But lower antibodies did not change how well the thyroid worked, how people felt, or how much thyroid medicine they needed. In people who already get enough selenium, extra pills did not help.
The C belongs to one of the two uses graded on this record. The other one carries its own letter.
As studied, and the trials
Replicated The same finding reproduced by a separate research group. A result seen once can be chance or bias; a replicated one is harder to explain away. on the antibody surrogate. Wichman et al. (Thyroid, 2016; PMID PubMed ID: the permanent number the US National Library of Medicine gives a published paper. Each one links to that paper's record on PubMed. 27702392), 16 trials; Huwiler et al. (Thyroid, 2024; PMID 38243784), 35 studies, moderate certainty; and Zhang et al. (Medicine (Baltimore), 2025; PMID 40898469), 21 randomized trials, all found thyroid peroxidase antibodies lower on selenium, and the 412-patient CATALYST trial found them lower at 12 months (Larsen et al., Eur Thyroid J, 2024; PMID 38215286). What did not change: quality of life, CATALYST's primary outcome, improved equally on placebo An inactive substitute made to look like the real treatment, so participants cannot tell which one they received.; TSH, thyroid ultrasound and quality of life in untreated patients were unchanged in a systematic review A review that searches for every study on a question using a method set in advance, then assesses them together. (Winther et al., Endocrine, 2017; PMID 27683225); heterogeneity How much the results of the pooled trials differ from each other. High heterogeneity makes a combined estimate less reliable. is extreme and several trials are null. Replication on a surrogate can carry B only when the surrogate is an established stand-in for a clinical outcome, and thyroid peroxidase antibodies are not one; whether their fall changes any clinical outcome remains to be shown. The pooled trials mixed selenomethionine, selenite and selenium yeast. Re-graded C by the editor in October 2026: thyroid peroxidase antibodies are not an established stand-in for a clinical outcome, and their fall has not changed thyroid function, levothyroxine dose or quality of life.
As studied, and the trials
Tested at scale and no benefit, with a diabetes signal. SELECT gave 200 mcg/day L-selenomethionine, this record's form, to 35,533 men and found no reduction in prostate or any other cancer, with a nonsignificant rise in type 2 diabetes (Lippman et al., JAMA, 2009; PMID 19066370). The Cochrane review found high-certainty evidence of no cancer benefit and a slight diabetes increase (Vinceti et al., 2018; PMID 29376219). The Nutritional Prevention of Cancer trial, run in low-selenium areas, missed its skin cancer primary (Clark et al., 1996; PMID 8971064) and found more type 2 diabetes on selenium, concentrated in those with the highest baseline levels (Stranges et al., 2007; PMID 17620655); Sel/Cel found no adenoma prevention and a diabetes signal in older participants (Thompson et al., 2016; PMID 27530657). For a trace mineral, more is not better. D on Route 2 Human randomized trials have been run and did not show benefit on the stated outcome. One of the two routes to the lowest letter on EBL's scale.; the diabetes signal is stated as a signal, since in the largest trial it did not reach significance.
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A letter describes the human evidence, not the product, the manufacturer, or the ingredient's worth.
See the full evidence record →Thyroid peroxidase antibodies lower in autoimmune thyroiditis across pooled trials, a surrogate; quality of life and TSH unchanged.
How this letter has changed · latest: October 2026 · Downgraded
Antibody lowering replicates across meta-analyses A study that pools the numbers from several trials into one combined estimate. (Huwiler et al., Thyroid, 2024; PMID 38243784) and the CATALYST trial (Larsen et al., Eur Thyroid J, 2024; PMID 38215286), but thyroid peroxidase antibodies are not an established stand-in for a clinical outcome, and quality of life, CATALYST's primary outcome, improved equally on placebo. The editor, who had signed the B in October, returned it to C the next day on the strict reading of the B definition (Arnold Sandlow, D.C.).PMID 38215286 PMID 38243784
Three pooled analyses of randomized trials agree that selenium lowers thyroid peroxidase antibodies: Wichman et al. (Thyroid, 2016; PMID 27702392), Huwiler et al. (Thyroid, 2024; PMID 38243784; 35 studies, moderate certainty) and Zhang et al. (Medicine, 2025; PMID 40898469). Replicated randomized evidence on a surrogate is B; the antibody titer, with no shown link to symptoms or thyroid function, is what keeps it from A, and the record states that no pooled analysis shows a clinical change. The headline, previously C on mixed cancer and cardiovascular trials, follows the best use to B; those trials keep their own rows and letters. Applied the written grade definition under the editor's standing instruction of October 2026 (Arnold Sandlow, D.C.).PMID 27702392 PMID 38243784 PMID 40898469
The headline ref argued its letter from cancer and cardiovascular trials while the title named thyroid antibodies, cited a selenium plus coenzyme Q10 trial (Alehagen et al., Int J Cardiol, 2013; PMID 22626835) as selenium evidence, and called the Nutritional Prevention of Cancer trial suggestive when its skin cancer primary was null (Clark et al., JAMA, 1996; PMID 8971064). The ref now follows the graded use. The replete-adult row rested on one secondary analysis; SELECT (Lippman et al., JAMA, 2009; PMID 19066370), the form-matched 35,533-man trial, the Cochrane review (Vinceti et al., 2018; PMID 29376219) and Sel/Cel (Thompson et al., 2016; PMID 27530657) are its evidence now, with the badge restated as no benefit and the diabetes excess stated as a signal. The thyroid row adds the null clinical outcomes: quality of life unchanged in CATALYST (Larsen et al., 2024; PMID 38215286) and TSH, ultrasound and quality of life unchanged in untreated patients (Winther et al., 2017; PMID 27683225). The form note says the antibody effect is pooled across three selenium forms. Letters unchanged. Applied the written grade definition under the editor's standing instruction of October 2026 (Arnold Sandlow, D.C.).PMID 19066370 PMID 22626835 PMID 27530657 PMID 27683225 PMID 29376219 PMID 38215286 PMID 8971064
Every change to a letter on this site is dated and published. The full evidence log →
Grade scope · Grade C overall, on autoimmune thyroiditis. Lowering of thyroid peroxidase antibodies in autoimmune thyroiditis is Grade C: three pooled analyses of randomized trials agree (Wichman, Thyroid 2016; Huwiler, Thyroid 2024; Zhang, Medicine 2025), but the antibody titer is not an established stand-in for a clinical outcome and has no shown link to symptoms or thyroid function. The cancer and cardiovascular trials are mixed and carry no letter of their own. Long-term supplementation in already-replete adults is Grade D (tested and found to cause harm rather than benefit; for a trace mineral, more is not better).
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Grade C reflects replicated randomized evidence on a marker that is not an established stand-in for a clinical outcome. Three pooled analyses agree that selenium lowers thyroid peroxidase antibodies, but an antibody titer is not a clinical outcome, and no pooled analysis shows a change in symptoms, thyroid hormone levels or thyroid volume. Under the written definition, B on a biomarker requires a marker such as HbA1c A blood test that reflects average blood sugar over the previous two to three months., LDL Low-density lipoprotein cholesterol, the cholesterol fraction most closely tied to plaque in the arteries., blood pressure or bone density, however many trials agree. EBL re-grades when new randomized evidence is published.
55-200 mcg/day. RDA is 55 mcg. Most intervention studies used 100-200 mcg/day. Background dietary intake varies substantially by region (soil-dependent).
The thyroid trials pooled selenomethionine, sodium selenite and selenium-enriched yeast, so the graded antibody effect is pooled across forms; one 2025 pool reports selenomethionine outperforming the others. The large cancer prevention trials used L-selenomethionine (SELECT) or selenium yeast. Selenium from food and supplements counts together, and the content of foods such as Brazil nuts varies widely.
The cited trials gave a daily dose (PMID 8971064, PMID 19066370, PMID 22626835, PMID 17620655). They do not state a time of day or whether doses were taken with food.
Checked against the full report for 1 of the 4 cited studies, and the published summary for the rest. What the cited trials did, not a schedule. Every supplement, by trial timing →
How to buy this: quality criteria, not a brand
55–200mcg/day. The tolerable upper intake level is 400 mcg from all sources.
Selenomethionine (organic, ~90% absorbed) over selenite (~50%).
Selenomethionine form stated; third-party tested.
Selenite; stacking a supplement with daily Brazil nuts (toxicity risk); exceeding 400mcg.
The grade and these criteria are set only from human trials. Any purchase link is convenience and does not affect the grade.
Anyone already selenium-replete, which in North America is most people eating an ordinary diet. Selenosis appears above roughly 400 mcg/day: brittle nails, hair loss, garlic breath, nausea. The therapeutic window here is one of the narrowest of any supplement on this site. From the trials: more type 2 diabetes on 200 mcg/day in the Nutritional Prevention of Cancer trial and a slight increase in the Cochrane pool (PMID 17620655, PMID 29376219); alopecia and dermatitis in SELECT (PMID 19066370); more adverse effects on selenium in one thyroid pool (PMID 27702392) and effects comparable to control in a later one (PMID 38243784).
The concern that belongs at the top of this entry is not a drug interaction. Pooled randomized evidence links selenium supplementation to a higher risk of type 2 diabetes, which is a poor trade for anyone taking it speculatively. Anticoagulants: possible additive effect. Chemotherapy and radiotherapy: antioxidant supplementation during treatment is an oncology decision.
Selenium supplementation and type 2 diabetes risk. Pooled randomized evidence links selenium supplementation to a higher risk of type 2 diabetes, which is the risk that belongs at the top of this entry rather than a drug interaction. Mao et al. (Endocrine, 2014; PMID 24858736): meta-analysis of randomized controlled trials People are assigned to the treatment or a comparison by chance, so the groups differ only in what they received. It is the design that can show cause and effect. of selenium supplementation and the risk of type 2 diabetes mellitus. Vinceti et al. (European Journal of Epidemiology, 2018; PMID 29974401): systematic review and meta-analysis of selenium exposure and the risk of type 2 diabetes.
If selenium status is uncertain or supplementation is being used to correct a deficiency, serum or plasma selenium can help document status. Thyroid testing should be guided by the clinical context. If selenium status is already adequate, increasing the dose further should not be treated as a treatment target.
Primary evidenceEleven papers, each linked to its PubMed record.
Thyroid, the graded use. Wichman et al. (Thyroid, 2016; PMID 27702392): 16 controlled trials; thyroid peroxidase antibodies lower at 3, 6 and 12 months in patients on levothyroxine and at 3 months in untreated patients, quality of evidence generally low, more adverse effects on selenium. Huwiler et al. (Thyroid, 2024; PMID 38243784): 35 studies; antibodies and TSH lower, moderate certainty overall, adverse effects comparable to control. Zhang et al. (Medicine (Baltimore), 2025; PMID 40898469): 21 randomized trials confirming the antibody fall, with selenomethionine outperforming other forms. Larsen et al. (Eur Thyroid J, 2024; PMID 38215286), CATALYST: 412 patients with autoimmune thyroiditis on levothyroxine, 200 mcg/day as selenium-enriched yeast for 12 months; quality of life, the primary outcome, improved equally on selenium and placebo. Winther et al. (Endocrine, 2017; PMID 27683225): systematic review; no change in TSH, thyroid ultrasound or quality of life in untreated patients, very low to low certainty. Replete adults, the D row. Clark et al. (JAMA, 1996; PMID 8971064), the Nutritional Prevention of Cancer trial: 1,312 patients with prior skin cancer in low-selenium areas of the eastern United States, 200 mcg/day; the primary endpoints The one outcome a trial names in advance as its main test. Results on other outcomes carry less weight., basal and squamous cell skin cancer, were not reduced, and the secondary cancer findings were not confirmed afterward. Stranges et al. (Ann Intern Med, 2007; PMID 17620655): the same trial's secondary analysis, type 2 diabetes more frequent on selenium (hazard ratio How often an event happened in the treated group compared with the control group over the same period. A ratio of 0.80 means 20 percent fewer events. 1.55), concentrated in the highest tertile of baseline plasma selenium. Lippman et al. (JAMA, 2009; PMID 19066370), SELECT: 35,533 men, 200 mcg/day L-selenomethionine, this record's form; no reduction in prostate or any other cancer, with a nonsignificant rise in type 2 diabetes. Thompson et al. (J Natl Cancer Inst, 2016; PMID 27530657), Sel/Cel: 1,621 adults after adenoma removal, 200 mcg/day as selenized yeast; no adenoma prevention and a diabetes signal in older participants. Vinceti et al. (Cochrane, 2018; PMID 29376219): high-certainty evidence of no effect on cancer incidence (RR 1.01), a slight increase in type 2 diabetes, and no clear influence of baseline selenium status. Alehagen et al. (2013; PMID 22626835) gave selenium combined with coenzyme Q10 and cannot grade selenium alone; context only. Grade C, the thyroid use, with the headline following it; replete-adult supplementation D on Route 2.
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These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. The grade on this page describes the strength of published human trial evidence in the populations studied. It is not a recommendation for any individual and is not medical advice. Starting, stopping or changing any supplement or medication is a decision for your physician.
Trials report an average. This box says whether any trial on this record looked at who the result applies to, and what it found.
For Long-term supplementation in already-replete adults, Grade D · Subgroup tested, effect differedIn the Nutritional Prevention of Cancer trial, the extra type 2 diabetes on selenium was concentrated in those with the highest baseline selenium levels (Stranges et al., 2007; PMID 17620655). A subgroup result shows who the effect appeared in within these trials. It does not change the letter.
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selenium[tiab] AND (thyroiditis[tiab] OR "thyroid peroxidase"[tiab] OR TPO[tiab]) AND (meta-analysis[pt] OR randomized controlled trial[pt])Grades move when new trials publish. A free monthly email lists every grade EBL moved, with the reason and the trial behind it: the sign-up is on the Evidence Log →