Selenium
Updated: Sep 25
Selenium – The Antioxidant Mineral
Selenium is a powerful trace mineral that supports thyroid health, immune function, and antioxidant protection. It helps your body fight oxidative stress, maintain a healthy metabolism, and reduce inflammation.
Why Selenium Is Important
• Supports Thyroid Function – Selenium is needed to convert inactive thyroid hormone (T4) into its active form (T3). Having good levels of T3 is crucial because it is part of a process that activates and recycles many other vitamins (such as B1, B2, B3, B6, B9, B12, vitamin A, and Vitamin K). Without enough T3, these vitamins will not function effectively.
• Antioxidant Protection – It's a key component of antioxidant enzymes that protect cells from damage.
• Immune Health – Selenium boosts the immune system and helps fight infections.
• Fertility & Hormone Balance – Plays a role in sperm quality and reproductive health.
Signs of Selenium Deficiency
Selenium deficiency can occur because many regions in the world have selenium-poor soil (such as the UK), and due to a poor diet and gut absorption issues.
Symptoms may include:
• Fatigue or muscle weakness
• Brittle hair or nails
• Slow thyroid function (hypothyroidism)
• Low immunity and frequent infections
• Brain fog or poor concentration
Recommended Daily Intake (RDI)
• Adults: 55 mcg/day
• Pregnant Women: 60 mcg/day
• Breastfeeding Women: 70 mcg/day
(Note: Selenium is required in tiny amounts — both deficiency and excess can cause problems.)
Top Food Sources of Selenium (No Brazil Nuts)
Tuna (yellowfin): 100g cooked = 92 mcg
Sardines: 100g canned = 52 mcg
Chicken breast: 100g cooked = 31 mcg
Turkey: 100g cooked = 27 mcg
Eggs: 1 large = 20 mcg
Cottage cheese: 1 cup (220g) = 20 mcg
Sunflower seeds: 30g (¼ cup) = 19 mcg
Brazil nuts are one of the richest sources of selenium; however, most of it is in the form of selenomethionine. To use selenium in selenomethionine efficiently, your body needs a healthy methylation pathway. If this system is slow or blocked, your body may struggle to process selenomethionine into the active forms that it needs, and so not benefit from it.
Selenite vs. Selenomethionine
Selenium is essential for making functional thyroid enzymes, but the form used determines how quickly and directly it reaches those enzymes:
The Methionine Trap (Selenomethionine): Selenomethionine is an organic form bound to an amino acid. Because the body mistakes it for standard methionine, it gets randomly incorporated into structural proteins (like muscle and albumin) throughout the body. The selenium remains "trapped" until those structural proteins are eventually broken down, delaying its functional availability to the thyroid.
Direct Pathway (Sodium Selenite): Sodium selenite is an inorganic salt that completely bypasses general protein building. Upon absorption, it reacts directly with glutathione to form hydrogen selenide, the direct, obligate precursor for building active selenocysteine enzymes.
How it works in the Thyroid
When provided in their direct forms, iodide and selenite work as a tightly coordinated team inside thyroid tissue:
Hormone Synthesis: When Iodide enters thyroid cells, the enzyme thyroid peroxidase (TPO) uses hydrogen peroxide to bind iodide to thyroglobulin, building thyroxine (T4).
Antioxidant Protection: This hydrogen peroxide is highly corrosive. Selenite immediately fuels Glutathione Peroxidase (GPx), an enzyme that acts as a biological fire extinguisher, neutralising excess hydrogen peroxide into water and protecting the thyroid gland from autoimmune inflammatory damage.
3. Hormone Activation: Selenite rapidly supplies the deiodinase enzymes required to strip an iodine atom from inactive T4 to convert it into active T3 in the liver and target tissues.
Iodide supplies the essential raw fuel without gut irritation, while selenite fuels the enzymatic machinery that cleans up the toxic oxidative byproducts and converts the hormone into its active state.
Why Keeping a 5-Day Food Diary is So Helpful (Especially with a Free App Like Cronometer)
If you've ever wondered whether you're getting enough vitamins and minerals from your diet, keeping a food diary for just 5 days can be a real eye-opener. Using a free app like Cronometer makes the process simple and surprisingly insightful.
Cronometer doesn't just track calories — it gives you a full nutritional breakdown of the food you eat each day. You’ll quickly see where your diet is strong and where you might be missing out on important nutrients like B12, magnesium, or iron.
One of the best things about using an app like this is that you can experiment. Try swapping in nutrient-dense foods — such as chicken liver, beets, or romaine lettuce — and see how even small changes can dramatically improve your nutrient intake.
If you're using my example one-day meal plans for specific vitamins and minerals, keep in mind: they're just examples to show what's possible. Cronometer helps you take it a step further by tailoring things to your own diet, preferences, and needs.
Tracking your eating habits for just 5 days can give you a clearer picture of how they support your health — and where a few smart tweaks can make all the difference.
Example Meal Plan to Meet the RDI
Here's an example of how to get ~70–90 mcg of selenium without Brazil nuts:
Breakfast:
2 boiled eggs (40 mcg)
1 cup cottage cheese (20 mcg)
Lunch:
Grilled chicken breast (100 g – 31 mcg)
Mixed salad with sunflower seeds (30 g – 19 mcg)
Snack:
1 small tin of sardines (50 g – 26 mcg)
Dinner:
Baked salmon or tuna steak (100 g – 92 mcg)
Steamed vegetables
Total: ~140–150 mcg selenium (well over the RDI, but still safe)
Selenium Supplements – Do You Need Them?
Most people can meet their nutritional needs through a balanced diet, especially if they include foods like fish, eggs, and seeds.
Vegans are at risk of deficiency in regions where the soil is selenium-deficient, such as the UK.
Supplements are usually not necessary unless you have a restricted diet or a confirmed deficiency.
Warning:
Taking too much selenium can generate oxidative stress, causing issues like hair loss, nausea, or a metallic taste in the mouth — so avoid high-dose supplements unless advised by a professional.
Hashimoto's, Graves', or HIV are conditions where supplementation may be beneficial. Most studies have used 200 mcg/d. One study showed a 40 % reduction in antibody levels after selenium supplementation, with 25% of patients completely normalising their antibody levels.
The selenium should be as Selenite salt, but should not be Selenomethionine or Selenium chelates, or Selenium amino acids complex, each of which has been shown not to work by Dr G. Russell-Jones' research (see B12oil.com for more information).
There must be no divalent cations in the formulation (such as calcium, magnesium, iron, zinc, copper or manganese) as these bind to the Selenite, forming insoluble compounds that we are unable to absorb.
The Best Supplements
To avoid the absorption issues, there is a topical skin oil available at B12oils.com
Metabolics Ionic Selenium Liquid (10% discount link)
Other brands that make a suitable selenium supplement without ingredients that stop absorption:
Vitamaze, Watspm Nutrition, Biosyn Senenase Peroral, Kyberg Orthodoc Selen, Selen Natrium -Selenit, Cafasel Nutri, KIKI Health.
Key Takeaways (Cheat Sheet)
• RDI: 55 mcg/day for adults
• Best Sources: Tuna (100g = 92 mcg), Sardines (100g = 52 mcg), Chicken breast (100g = 31 mcg), Eggs (1 egg = 20 mcg), Sunflower seeds (30g = 19 mcg)
• Deficiency Signs: Fatigue, brittle hair/nails, low immunity, thyroid issues
• Tip: A portion of tuna or two eggs + chicken easily meets daily needs
For more information, please visit https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/
This information is provided for educational purposes only and should not be considered medical advice.
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