Last revised by Daniel J Bell on 1 Dec 2021

Selenium (chemical symbol Se) is one of the trace elements. It has a vital role in the metabolism of the thyroid hormones and for the functioning of several important enzymes.

Selenium has the atomic number 34 with an atomic weight of 78.96 g/mol. It is a non-metal and has similar properties to sulfur which lies above it in group VI of the periodic table. 

Six isotopes of selenium account for almost all the naturally occurring selenium on earth, these are Se-74, Se-76, Se-77, Se-78, Se-80 and Se-81 6.

Brazil nuts, seeds, green vegetables and certain mushrooms contain significant amounts of selenium in areas in which selenium is present in the soil. Fish and meat may also be good sources if the animals dwell in selenium-rich environments. The USA sets the maximal daily selenium allowance as 400 μg per day in adults, in order to avoid chronic toxicity (see below).

Selenium is essential for the synthesis of selenocysteine, a rare amino acid in which the normal sulfur of cysteine, has been substituted by selenium. This is needed for the production of twenty-five different selenoproteins (proteins containing selenium) coded for by the human genome 2.

Selenoproteins include 2:

  • D1, D2 and D3, are deiodinases required for the metabolism of the thyroid hormones
  • glutathione peroxidases (GPX): for cellular antioxidant defense
  • thioredoxin reductases

Selenium deficiency is well recognized, the two main diseases being

Selenosis, the condition caused by chronic selenium excess, results in deleterious effects on the skin, GI tract, and nervous system 2.

  • selenium-75 (Se-75) is used as a radiotracer for adrenal, parathyroid and parathyroid scintigraphy
  • the use of gadolinium contrast agents can result in spuriously raised selenium levels by interfering with the spectrometric assays commonly used 3
  • selenium was discovered by Jöns Jakob Berzelius, a Swedish chemist, in 1817 4
  • selenium originates from the Ancient Greek word, σελήνη (selene), which means moon (named because it was discovered at the same time as tellurium, which means 'earth'!)

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