Selenium occurs in all tissues. Selenium works with vitamin E as an antioxidant. It helps protect cells against damage by free radicals, which are reactive by-products of normal cell activity. Selenium may help protect against some cancers. Selenium is also necessary for the thyroid gland to function normally.
Selenium deficiency is rare, even in New Zealand and Finland, where selenium intake is much lower than in the United States and Canada. In certain areas of China where selenium intake is even lower, people with selenium deficiency are more likely to develop Keshan disease, a viral disease that affects mainly children and young women. Keshan disease damages the walls of the heart, resulting in cardiomyopathy.
Growing children with selenium deficiency may develop a slowly progressive, disabling disorder of the joints and bone (Kashin-Beck disease). This disease is common in Siberia and China.
Selenium deficiency may work with iodine deficiency to cause a goiter and an underactive thyroid gland (hypothyroidism) in people who have both deficiencies.
Doctors suspect selenium deficiency based on the person’s circumstances and symptoms. Blood tests for this deficiency are not readily available.
Treatment with a selenium supplement may result in a complete recovery. Taking selenium supplements can prevent but not cure cardiomyopathy due to Keshan disease.
Taking more than 900 micrograms of a nonprescription selenium supplement each day can have harmful effects. Symptoms include nausea, vomiting, diarrhea, hair loss, abnormal nails, a rash, fatigue, and nerve damage. The breath may smell like garlic.
The diagnosis of selenium excess is based on symptoms, particularly rapid hair loss.
Treatment of selenium excess involves reducing selenium consumption.