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.

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.

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.

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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.

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

Sources: Selenium in diet: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); GPX1: Glutathione peroxidase 1 (human) (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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