Deiodinases are able to extract electrons from iodides, and iodides from iodothyronines. They are, thus, involved in thyroid-hormone regulation, participating in the protection of thyrocytes from damage by H2O2 produced for thyroid-hormone biosynthesis. About 200 million years ago, new selenoproteins were developed as mammalian GSH-Px enzymes. Nutritional sources of selenium
Dietary selenium comes from meat, nuts, cereals and mushrooms. Brazil nuts are the richest dietary source (though this is soil-dependent, since the Brazil nut does not require high levels of the element for its own needs).The US Recommended Dietary Allowance (RDA) of selenium for teenagers and adults is 55 µg/day. Selenium as a dietary supplement is available in many forms, including multi-vitamins/mineral supplements, which typically contain 55 or 70 µg/serving. Selenium-specific supplements typically contain either 100 or 200 µg/serving. In June 2015, the US Food and Drug Administration (FDA) published its final rule establishing the requirement of minimum and maximum levels of selenium in infant formula.The selenium content in the human body is believed to be in the 13–20 mg range. Indicator plant species
Certain species of plants are considered indicators of high selenium content of the soil because they require high levels of selenium to thrive. The main selenium indicator plants are Astragalus species (including some locoweeds), princes plume (Stanleya sp. ), woody asters (Xylorhiza sp. ), and false goldenweed (Oonopsis sp.)
Selenous acid can also be made directly by oxidizing elemental selenium with nitric acid:

  
    
      
        
          3
          
          Se
          +
          4
          
          
            HNO
            
              3
            
            
              
            
          
          +
          
            H
            
              2
            
            
              
            
          
          O
          ⟶
          3
          
          
            H
            
              2
            
            
              
            
          
          
            SeO
            
              3
            
            
              
            
          
          +
          4
          
          NO
        
      
    
    {\displaystyle {\ce {3 Se + 4 HNO3 + H2O -> 3 H2SeO3 + 4 NO}}}
  Unlike sulfur, which forms a stable trioxide, selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185 °C:

  
    
      
        
          2
          
          
            SeO
            
              3
            
            
              
            
          
          ⟶
          2
          
          
            SeO
            
              2
            
            
              
            
          
          +
          
            O
            
              2
            
            
              
            
          
        
      
    
    {\displaystyle {\ce {2 SeO3 -> 2 SeO2 + O2}}}
   (ΔH = −54 kJ/mol)Selenium trioxide is produced in the laboratory by the reaction of anhydrous potassium selenate (K2SeO4) and sulfur trioxide (SO3).Salts of selenous acid are called selenites.