Serum selenium levels can be used in the determination of deficiency or toxicity. Plasma and serum contains 75 percent of the selenium measured in whole blood and reflects recent dietary intake. Selenium deficiency can occur endemically or as a result of sustained TPN or restricted diets and has been associated with cardiomyopathy and may exacerbate hypothyroidism. Selenium toxicity is relatively rare. Excess intake of selenium can result in symptoms consistent with selenosis and include gastrointestinal upset, hair loss, white blotchy nails, and mild nerve damage.