Selenium deficiency

**Definition:** Selenium deficiency is rare but has been observed in individuals on long-term total parenteral nutrition lacking selenium. Clinical manifestations of deficiency arising from such situations are uncommon and poorly defined. They include muscular weakness and myalgia with, in several instances, the development of congestive heart failure. The importance of selenium for thyroid hormone metabolism is evident from changes in the T3–T4 ratio which develop after relatively mild selenium depletion in infants and elderly subjects.