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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# Selenium

Selenium is part of antioxidant enzymes and of the enzymes that activate thyroid hormone; both lack and excess are harmful.

Updated 2026-11-01

## Key facts

- It enters proteins as selenocysteine, the 21st amino acid, encoded by a UGA codon (normally a stop codon).
- Glutathione peroxidases: remove peroxides using glutathione; they work alongside vitamin E.
- Deiodinases: convert T4 to T3 (active) or reverse T3 (inactive).
- Deficiency: Keshan disease (cardiomyopathy) and Kashin-Beck disease (osteoarthropathy) in areas of China with poor soils. Excess (selenosis): hair and nail loss, garlic breath, neuropathy.

## In detail

- Sources: Brazil nuts (very rich), fish and shellfish, meat, eggs, cereals; the content of plant foods depends on the soil they grew in.
- Other selenoproteins: thioredoxin reductase (regenerates thioredoxin) and selenoprotein P (carries selenium in plasma).
- Glutathione peroxidase reduces hydrogen peroxide and lipid hydroperoxides; glutathione reductase regenerates oxidised glutathione with NADPH.
- Causes of deficiency: poor soils, long-term parenteral nutrition without selenium, malabsorption, dialysis.
- Deficiency worsens the effect of iodine lack on the thyroid and increases oxidative damage.
- Lab: plasma or serum selenium; glutathione peroxidase activity.

**Also called**
Se, Selenocysteine

## Clinical notes

- Keshan disease is a cardiomyopathy of children and young women in regions with selenium-poor soil, preventable with supplements.
- Selenium is added to long-term parenteral nutrition to prevent cardiomyopathy and muscle weakness.
- The range between need and toxicity is narrow: large amounts of Brazil nuts or badly formulated supplements can cause selenosis.

## Related conditions

- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)
- [Heart failure](https://durgical.com/en/conditions/heart-failure/)

## Related

- [Vitamin E (α-tocopherol)](https://durgical.com/en/biochemistry/molecules/vitamin-e/)
- [Iodine](https://durgical.com/en/biochemistry/topics/iodine/)

## Test yourself

Think of the answer, then tap the question to see it.

## How is selenium built into proteins?

As selenocysteine, encoded by a UGA codon that a special mRNA structure turns from a stop into an insertion signal.

## Which antioxidant enzymes contain selenium?

Glutathione peroxidases and thioredoxin reductase.

## How is selenium linked to the thyroid?

The deiodinases that convert T4 into active T3 are selenoproteins.

## What is Keshan disease?

A cardiomyopathy caused by selenium deficiency in areas of China with very selenium-poor soils.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Selenium in diet: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002414.htm) (MedlinePlus, US National Library of Medicine, 2026); [GPX1: Glutathione peroxidase 1 (human)](https://www.uniprot.org/uniprotkb/P07203/entry) (UniProt, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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