Selenium: The Overlooked Mineral Your Thyroid Depends On

Why Selenium Is Underrated

Selenium is an essential trace mineral incorporated into selenoproteins — proteins with specific biological roles. The most important:

- Glutathione peroxidases (GPx): Major antioxidant enzymes that neutralise reactive oxygen species - Thioredoxin reductases (TrxR): Regulates cell growth and the redox state of cells - Iodothyronine deiodinases: Enzymes that convert T4 (inactive thyroid hormone) to T3 (active form) - Selenoprotein P: Plasma selenium transport protein, protects blood vessel lining

The UK Selenium Problem

UK and Irish soil selenium levels are among the lowest in Europe, significantly lower than North American soils. The main reason: European soils were glacially leached and Europe imports less selenium-enriched crops from North America than it once did.

The result: UK average selenium intakes have fallen from ~60–70mcg/day in the 1970s (when more North American wheat was imported) to ~30–40mcg/day in 2020. The UK RNI is 55–75mcg/day.

People on plant-based diets — with no selenium-rich seafood or meat — are particularly at risk.

Selenium and the Thyroid

The thyroid contains the highest concentration of selenium per gram of any organ in the body. Selenium is required at multiple steps:

1. T4 → T3 conversion: Deiodinase enzymes require selenium. Without it, inactive T4 cannot be converted to active T3. 2. Protection from oxidative damage: Hydrogen peroxide is generated during thyroid hormone synthesis — selenoproteins neutralise it. 3. Hashimoto's autoimmunity: A 2003 meta-analysis found 200mcg selenium supplementation daily reduced anti-TPO antibody titers significantly in Hashimoto's patients. Recent 2021 evidence is more mixed, but the association between low selenium and thyroid autoimmunity is consistent.

If you have thyroid symptoms and have ruled out iodine deficiency, consider testing selenium status.

Selenium and Immunity

Selenium deficiency impairs both innate and adaptive immune responses: - Reduced NK cell activity - Impaired T-cell proliferation - Reduced antibody production after vaccination (selenium deficiency reduces vaccine immunogenicity)

A 2022 study found low selenium was associated with worse COVID-19 outcomes — a finding consistent with its role in viral immunity.

Selenium and Cancer Prevention

Selenium's antioxidant role has made it a focus of cancer prevention research. The picture is complicated:

- Prostate cancer: SELECT trial (negative) showed selenium didn't prevent prostate cancer in selenium-sufficient men. Earlier NPC trial (positive) showed benefit — but the populations differed in baseline selenium status. - Colorectal cancer: Some evidence of benefit from selenium status. - Conclusion: Benefit is largely in correcting deficiency. Megadosing above sufficiency doesn't add protection and may have adverse effects.

Selenium-Rich Foods

| Food | Selenium content | |------|-----------------| | Brazil nuts (1–2 nuts) | 50–90mcg | | Tuna, canned (100g) | 90–100mcg | | Sardines (100g) | 45–60mcg | | Egg (1 large) | 15–20mcg | | Chicken breast (100g) | 20–30mcg | | Wholemeal bread (2 slices) | 10–15mcg (UK soil dependent) | | Brazil nuts are by far the most concentrated dietary source. |

Important caveat on Brazil nuts: Selenium content is extremely variable (8–83mcg per nut). Daily consumption is not recommended due to risk of oversupply — 1–2 Brazil nuts per day is a rough practical target.

Supplementation

Selenomethionine is the preferred supplemental form — organically bound selenium with the best bioavailability and tissue retention.

Dose: 55–200mcg/day. Most supplements contain 100–200mcg. Do not exceed 400mcg/day (the established upper tolerable intake). Toxicity signs at very high chronic intake: hair and nail brittleness, garlic breath odour.

Testing: Plasma selenium (normal range 75–120 μg/L in the UK). Whole-blood selenium is a better indicator of long-term status.

Who Should Consider Supplementing

- People who don't eat fish, meat, or eggs regularly - Anyone with thyroid autoimmunity (Hashimoto's) — discuss with GP first - People on strict plant-based diets with no Brazil nut consumption - Those in selenium-depleted regions (most of the UK)

Selenium at supplemental doses should be discussed with your GP if you have thyroid disease. Don't self-supplement above 200mcg without testing.