SeleniumMineral

• Cancer riskNot shown to help

  The SELECT trial is the decisive one here: 35,533 men took 200 mcg/day selenomethionine, with or without vitamin E, for about 5.5 years [3].
  
  It found no reduction in prostate cancer risk [4]. A 2018 Cochrane review of 83 studies landed in the same place — little to no effect on cancer incidence or mortality [1].
  
  That Cochrane finding is rated high-certainty for prostate, colorectal, lung, and bladder cancer specifically [1].
  
  The older signal has not vanished, though. The NPC trial's secondary analysis found 49% lower prostate cancer risk among men who started with the lowest blood selenium [1].
  
  Observational cohorts separately link top-versus-bottom selenium status to 28% lower cancer risk and 24% lower cancer mortality [1].
  
  We are not resolving that for you, but the most common reading is that baseline status is doing the work — topping up someone already replete changed nothing [1].
  
  The FDA permits only heavily qualified health claims linking selenium and cancer, and describes the evidence as limited [1]. ^[3]

• Hashimoto's and autoimmune thyroiditisPossibly helps

  A 16-trial meta-analysis pooling 1,494 adults found that 80–200 mcg/day for 3 to 12 months lowered thyroid peroxidase antibody levels, in people already taking levothyroxine [1].
  
  Selenomethionine outperformed sodium selenite in those trials, possibly on absorption [6].
  
  A separate 9-trial review of 679 participants found no improvement in TSH, thyroid ultrasound findings, or quality of life from selenium alone [1].
  
  Both sit inside the same NIH ODS review, so this is genuinely mixed clinical evidence rather than a source-quality gap.
  
  The Cochrane summary rates the underlying trial quality as unclear-to-high risk of bias [7]. ^[1]

• Cardiovascular eventsNot shown to help

  A Cochrane review combining SELECT with 11 other trials — 19,715 adults in total — found no reduction in fatal or nonfatal cardiovascular events [1].
  
  One finding cuts the other way, and it is worth naming precisely. In a 43-trial meta-analysis, antioxidant formulas containing selenium were linked to 23% lower cardiovascular mortality than formulas without it [1].
  
  That compares multi-ingredient products against each other, not selenium against placebo. It says nothing about what a selenium capsule alone does. ^[1]

• Dementia and Alzheimer's diseaseNot shown to help

  The PREADVISE trial followed 7,540 men for 5.4 years on 200 mcg/day selenium [1].
  
  It found no reduction in the incidence of dementia or Alzheimer's disease [1]. ^[1]

• Depressive symptomsPossibly helps

  A 2022 systematic review and meta-analysis of 15 studies found selenium supplementation lowered depressive symptom scores, at a weighted mean difference of -0.37 [11].
  
  Higher selenium intake was also linked to lower postpartum depression risk [11].
  
  The same analysis found no significant correlation between people's actual blood selenium levels and how depressed they were [11].
  
  Its authors call the finding preliminary and ask for further research. Read it that way. ^[11]

• HIV: CD4 count and viral loadNot shown to help

  Two Cochrane reviews found selenium supplementation had little to no effect on CD4+ T-cell counts or viral load [1].
  
  A different outcome in the same population did move. One Nigerian trial reported 68% lower preterm birth risk among pregnant women with HIV who took selenium [1].
  
  That is one trial on a separate endpoint, not support for the immune-marker claim. ^[1]

• Male fertility and sperm motilityPossibly helps

  Baseline status appears to decide this one. A 3-month Scottish trial in men with low plasma selenium and reduced sperm motility found 100 mcg/day selenomethionine improved motility and conception odds versus placebo [1].
  
  A 48-week trial in 54 healthy, nonsmoking men at the higher dose of 300 mcg/day selenium yeast found no difference in sperm concentration or motility [1].
  
  NIH ODS's own conclusion is that there is not yet enough data to say selenium reliably helps male fertility [1]. ^[1]

• Keshan disease — an endemic cardiomyopathy first identified in 1935 in low-selenium regions of China, where average adult intake ran at 10 mcg/day or less [1]
• At least 20 mcg/day was the intake associated with lower Keshan disease risk, and a 2018 meta-analysis of 41 studies found supplementation cut that risk by 86% [1]
• Kashin-Beck disease — a form of osteoarthritis appearing in childhood or puberty in low-selenium, low-iodine areas of China, Tibet, Siberia, and North Korea [1]
• Worsening of an existing iodine deficiency, which raises the risk of congenital hypothyroidism in infants [1]
• Endemic myxedematous hypothyroidism, documented in regions such as Central Africa [1]
• Less firmly established, and described mainly in narrative reviews: fatigue, muscle weakness, weakened immune function, hair loss, mental fog, infertility, and low mood or anxiety [1]
• Deficiency is very rare in the US and Canada. Risk concentrates in people eating food grown in low-selenium soil, people on long-term hemodialysis, and people living with HIV [1]^[1]

• SelenomethionineThe form used in SELECT, and the one that outperformed sodium selenite at lowering thyroid antibodies

  Elemental content

  Organic selenium bound into an amino acid — also the form that predominates in food

  Absorption

  Absorbed at up to about 90%, the figure NIH ODS gives for all four common forms [1].
  
  DrugBank reads it as more readily absorbed than inorganic selenite [17].

  Best for

  The form used in SELECT, and the one that outperformed sodium selenite at lowering thyroid antibodies [1]

• Selenium-enriched yeastThe form used in the NPC cancer trial and in the 48-week male-fertility trial

  Elemental content

  Yeast grown in a high-selenium medium; the selenium ends up mostly as selenomethionine

  Absorption

  Up to about 90%, inside the same band NIH ODS reports for every common form [1].

  Best for

  The form used in the NPC cancer trial and in the 48-week male-fertility trial [1]

• Sodium seleniteCommon and cheap in multivitamin/mineral formulas, which usually carry around 55 mcg per dose

  Elemental content

  Inorganic selenium salt, not bound to an amino acid

  Absorption

  Up to about 90% by NIH ODS's general figure [1].
  
  In autoimmune thyroiditis trials it lowered thyroid antibodies less than selenomethionine did, possibly an absorption difference specific to that setting [1].

  Best for

  Common and cheap in multivitamin/mineral formulas, which usually carry around 55 mcg per dose [1]

• Sodium selenateNothing in the NIH ODS review singles this form out over the other three

  Elemental content

  The other inorganic salt, chemically distinct from selenite

  Absorption

  Also sits in the up-to-90% band NIH ODS reports [1].

  Best for

  Nothing in the NIH ODS review singles this form out over the other three [1]

• Selenosis from chronic excess

  Chronically high intake causes selenosis. Its most recognizable signs are hair loss and brittle or shedding nails [1].
  
  Also documented: garlic breath odor, a metallic taste, skin rash, nausea, diarrhea, fatigue, irritability, and nervous system abnormalities [1].
  
  The 400 mcg/day adult UL exists specifically to keep intake under that threshold [1]. ^[1]

• Mislabeled products have caused mass poisoning

  This is not hypothetical. A 2008 US outbreak traced to a mislabeled liquid supplement affected 201 people at a median dose of about 41,749 mcg/day, over 100 times the UL [12].
  
  Diarrhea hit 78% of them, fatigue 75%, hair loss 72%, joint pain 70%, nail discoloration or brittleness 61%, and nausea 58% [12].
  
  One person was hospitalized. Nail and fatigue symptoms were still present in over half of affected people at 90 days [12]. ^[12]

• Type 2 diabetes signal, unresolved

  Some RCTs, including the NPC trial's secondary analysis, found higher type 2 diabetes risk at around 200 mcg/day, concentrated in participants who already had higher selenium status [1].
  
  Other RCTs running 3 to 5 years did not replicate that [1].
  
  Meta-analyses of glycemic control suggest selenium likely does not shift fasting glucose or HbA1c [18]. The diabetes-incidence question itself stays open. ^[1]

• Pregnancy with thyroid antibodies

  The RDA rises in pregnancy (60 mcg/day) and lactation (70 mcg/day), while the UL stays at 400 mcg/day [1].
  
  For pregnant women who test positive for thyroid peroxidase antibodies specifically, the American Thyroid Association issued a weak recommendation against selenium supplementation, on moderate-quality evidence [1].
  
  A broader systematic review of selenium in pregnancy concluded that benefit and harm are both understudied, and called for more robust trials [22]. ^[1]

• Kidney dysfunction and malabsorption

  Selenium is partly excreted in urine, so people with renal dysfunction may need a dose adjustment [1].
  
  Malabsorption syndromes can reduce how much oral selenium is taken up, sometimes calling for larger oral doses or a parenteral route [1]. ^[1]

• Cisplatin chemotherapy

  Cisplatin lowers selenium levels in hair and serum, though whether that matters clinically is unclear [1].
  
  Small studies suggest selenium supplementation may reduce cisplatin's toxicity. A Cochrane review concluded the evidence that selenium eases chemotherapy side effects is insufficient [1]. ^[1]

• Levothyroxine

  A drug-interaction database found no direct pharmacological interaction listed between selenium itself and levothyroxine [16].
  
  The practical issue is the vehicle. Multivitamin-with-minerals products are advised to sit at least 4 hours apart from levothyroxine, since minerals cut its absorption [16].
  
  Separately, selenium has been studied as an add-on alongside levothyroxine in autoimmune thyroiditis, where several trials saw thyroid antibody levels fall [1]. ^[16]

• Statins — a hypothesis, not a confirmed interaction

  Statins have been hypothesized to affect selenoprotein synthesis, and observational work — mostly in Brazilian populations with low dietary selenium — has explored a link to statin-associated muscle symptoms [1].
  
  A small clinical trial of coenzyme Q10 plus selenium did not reduce atorvastatin-induced myopathy [1].
  
  No established clinical drug interaction has been confirmed in either direction [1]. ^[1]

Selenium in food is bound to protein, so the richest natural sources are seafood, meat, poultry, and organ meats [1].

Brazil nuts are in a category of their own. USDA FoodData Central puts them at 544 mcg per ounce, which is about 6 to 8 nuts [1].

Reported content per individual nut runs 68–96 mcg across sources. That spread is genuine natural variation by growing region and soil, not a factual disagreement [1].

From NIH ODS's food table, per typical serving: cooked yellowfin tuna (3 oz) 92 mcg, canned sardines (3 oz) 45 mcg, cooked shrimp (3 oz) 42 mcg [1].

A pork chop or beef bottom round (3 oz) gives 37 mcg, beef liver 28 mcg, roasted turkey 26 mcg, cod 24 mcg [1].

Lower down: a hard-boiled egg is 15 mcg, and a cup of cooked oatmeal or brown rice around 12–13 mcg [1].

Plant foods are the unpredictable ones. Their selenium tracks local soil concentration, while animal products stay steadier thanks to the animals' own regulation and standardized feed [1].

People following vegetarian or vegan diets may take in less than average [1].

Cooking removes a real fraction of it. In selenium-biofortified potato, boiling cut total selenium by about 43% and frying by about 32%, the latter through volatilization [20].

A separate study of African fresh foods found significant selenium losses from boiling and frying in most foods tested, plus further losses from soaking some foods overnight [21].

No trial has compared morning versus evening selenium dosing, so there is no evidence-backed best time [1].

Cleveland Clinic's consumer drug guidance says take capsules or tablets with a glass of water, with food, at consistent intervals [15].

It cites no trial data showing a fasted-versus-fed difference, and NIH ODS reports absorption up to about 90% without tying it to fasting state [1].

The one real timing rule here belongs to a different drug. Keep multivitamin-with-minerals products at least 4 hours away from levothyroxine [16].