High selenium levels in waterways were found to cause congenital disorders in oviparous species, including wetland birds and fish. Elevated dietary methylmercury levels can amplify the harm of selenium toxicity in oviparous species. Biological role
Although it is toxic in large doses, selenium is an essential micronutrient for animals. In plants, it occurs as a bystander mineral, sometimes in toxic proportions in forage (some plants may accumulate selenium as a defense against being eaten by animals, but other plants, such as locoweed, require selenium, and their growth indicates the presence of selenium in soil).Selenium is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient that functions as cofactor for reduction of antioxidant enzymes, such as glutathione peroxidases and certain forms of thioredoxin reductase found in animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium). The glutathione peroxidase family of enzymes (GSH-Px) catalyze certain reactions that remove reactive oxygen species such as hydrogen peroxide and organic hydroperoxides:

2 GSH + H2O2----GSH-Px → GSSG + 2 H2OThe thyroid gland and every cell that uses thyroid hormone use selenium, which is a cofactor for the three of the four known types of thyroid hormone deiodinases, which activate and then deactivate various thyroid hormones and their metabolites; the iodothyronine deiodinases are the subfamily of deiodinase enzymes that use selenium as the otherwise rare amino acid selenocysteine.
Whether a patient experiences pain or not largely depends on the location of the affected disc and the amount of pressure that is being put on the spinal column and surrounding nerve roots. Nevertheless, degenerative disc disease is one of the most common sources of back pain and affects approximately 30 million people every year. With symptomatic degenerative disc disease, the pain can vary depending on the location of the affected disc. A degenerated disc in the lower back can result in lower back pain, sometimes radiating to the hips, as well as pain in the buttocks, thighs or legs. If pressure is being placed on the nerves by exposed nucleus pulposus, sporadic tingling or weakness through the knees and legs can also occur. A degenerated disc in the upper neck will often result in pain to the neck, arm, shoulders and hands; tingling in the fingers may also be evident if nerve impingement is occurring. Pain is most commonly felt or worsened by movements such as sitting, bending, lifting and twisting. After an injury, some discs become painful because of inflammation and the pain comes and goes. Some people have nerve endings that penetrate more deeply into the anulus fibrosus (outer layer of the disc) than others, making discs more likely to generate pain. In the alternative, the healing of trauma to the outer anulus fibrosus may result in the innervation of the scar tissue and pain impulses from the disc, as these nerves become inflamed by nucleus pulposus material.