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

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.

In selenium deficiency, antioxidants are lacking in the heart and muscles. As a result, cardiomyopathy and muscle weakness may occur.

Doctors suspect selenium deficiency based on the person's circumstances and symptoms. Treatment with a selenium supplement may result in a complete recovery. Taking selenium supplements can prevent but not cure cardiomyopathy due to Keshan disease.

Selenium Excess

Taking more than 1 milligram 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 is based on symptoms, particularly rapid hair loss. Treatment involves reducing selenium consumption.

Last full review/revision August 2008 by Larry E. Johnson, MD, PhD