Selenium is a trace mineral and an antioxidant that is important for an array of cellular functions. Selenium is found in abundance in vegetables, fruits, and whole grains. I never recommend high doses of a single vitamin or mineral. It’s not only unnatural to our body’s biochemistry, but there is also evidence to suggest undesired effects maybe experienced. Selenium is a classic example of this. 
Supplementation with selenium in high doses has been shown to affect thyroid metabolism. Selenium promotes the activity of enzymes called deiodinases which convert the relatively inactive T4 thyroid hormone into the (five times) more active form, T3. In a study involving a group of men, those given large doses of selenium (297 micrograms) showed a clear drop in T3. These men also showed an increase in thyroid-stimulating hormone (TSH) which is a tell-tale sign that their bodies were attempting to compensate for the low-level of bioactive thyroid.
Disturbing the delicate thyroid metabolism axis can lead to a host of health problems, one of which is unwanted fat gain. If athletes eat the amount of fruits and vegetables that they should and use a quality multi-mineral complex such as MultiPro 32X, than individual supplementation with selenium should not be necessary.
Source: J. Nutr 133:3443-3448, 2005.
