Hydrogen sulfide reacts with aqueous selenous acid to produce selenium disulfide:

  
    
      
        
          
            H
            
              2
            
            
              
            
          
          
            SeO
            
              3
            
            
              
            
          
          +
          2
          
          
            H
            
              2
            
            
              
            
          
          S
          ⟶
          
            SeS
            
              2
            
            
              
            
          
          +
          3
          
          
            H
            
              2
            
            
              
            
          
          O
        
      
    
    {\displaystyle {\ce {H2SeO3 + 2 H2S -> SeS2 + 3 H2O}}}
  Selenium disulfide consists of 8-membered rings.
Selenium is a cofactor for the three thyroid hormone deiodinases, helping activate and then deactivate various thyroid hormones and their metabolites. Isolated selenium deficiency is now being investigated for its role in induction autoimmune reactions in thyroid gland in Hashimotos disease. However, in a case of combined iodine and selenium deficiency, selenium deficiency was shown to play a thyroid-protecting role. See also
Abundance of elements in Earths crust
ACES (nutritional supplement)
Selenium yeast

Notes
References
External links

Selenium at The Periodic Table of Videos (University of Nottingham)
National Institutes of Health page on Selenium
Assay Archived 2012-02-26 at the Wayback Machine
ATSDR – Toxicological Profile: Selenium
CDC – NIOSH Pocket Guide to Chemical Hazards
Peter van der Krogt elements site