Selenium has a number of important biological roles. It is a component of the enzyme glutathione peroxidase, which plays a crucial role in the body's antioxidant defense system. It is also required for the proper functioning of the thyroid gland and for the metabolism of iodine. Selenium deficiency can lead to a number of health problems, including impaired immune function and an increased risk of certain types of cancer. On the other hand, excessive intake of selenium can also be toxic.