Selenium deficiency is an important cause of Keshan disease. Selenium deficiency can induce liver necrosis and selenium deficiency can induce cardiovascular disease. In people with mild or moderate selenium deficiency, signs and symptoms are not obvious. Selenium is an important trace element for maintaining normal physiological functions of human body. Selenium is absorbed by the intestine after being ingested by the body and distributed in organs such as liver, spleen, kidney and heart. Selenium is combined with proteins and transported to tissues via blood, and selenium is involved in constituting glutathione peroxidase, which can catalyze the conversion of reduced glutathione into oxidized glutathione and prevent the damage of hydrogen peroxide and oxidized lipids to cells. Selenium deficiency is an important cause of Keshan disease. Selenium deficiency induces liver necrosis and selenium deficiency induces cardiovascular disease. In people with mild or moderate selenium deficiency, signs and symptoms are not obvious. How should selenium deficiency be diagnosed Selenium is an important trace element to maintain normal physiological functions of human body. Selenium is absorbed by the intestine after being ingested by the body and distributed in organs such as liver, spleen, kidney and heart. Selenium is combined with proteins and transported to tissues via blood, and selenium is involved in constituting glutathione peroxidase, which can catalyze the conversion of reduced glutathione into oxidized glutathione and prevent the damage of hydrogen peroxide and oxidized lipids to cells. Selenium deficiency can cause leukodystrophy in animals, and there is a correlation between selenium in the human environment and the level of selenium load in humans and the onset of Keshan disease. Supplementation of a certain amount of selenium to the population in the disease area can reduce the incidence of Keshan disease. In addition selenium has been shown to have anti-carcinogenic effects against a variety of carcinogens in animal experiments. However, excessive amount of selenium can cause selenosis and hair loss and nail loss. Selenium supplementation should not be overdosed. Because excessive intake of selenium can lead to poisoning, hair loss, demodex, etc. The amount of selenium in the diet in most areas of China is adequate and safe. The selenium poisoning caused by excess selenium seen clinically is divided into acute, subacute and chronic. The most important cause of poisoning is the direct or indirect intake and exposure of the body to a large amount of selenium, including occupational and geographical causes, dietary habits and drug abuse, etc. 1, acute selenium poisoning: acute poisoning usually occurs after ingestion of a large amount of high selenium substances, daily intake of selenium up to 400 ~ 800 mg/kg body weight can lead to acute poisoning. The main manifestations are abnormal movement and postural sickness, respiratory distress, gastric distention, high fever, rapid pulse, deficiency and death due to respiratory failure. Lethal poisoning death is mostly preceded by direct myocardial depression and intractable hypotension due to peripheral vasodilation. The characteristic symptoms are garlic or sour odor on the breath, nausea, vomiting, abdominal pain, restlessness, excessive salivation and muscle cramps. Children with acute selenium poisoning generally have dizziness, headache, weakness, drowsiness, nausea, vomiting, diarrhea, garlic odor of breath and sweat, and irritation of the upper respiratory tract and conjunctiva. In severe cases, there are manifestations of bronchitis, chills, high fever, profuse sweating, finger tremors, and hepatomegaly. Acute selenium toxicity is characterized by hair and nail loss, rash, occurrence of peripheral neuropathy, and mottled tooth color. Laboratory tests, leukocytosis, urinary selenium level is not high, and the symptoms gradually improve after 2 to 3 days. Those who mistakenly take sodium selenite, produce polyneuritis and myocarditis, should be distinguished from acute selenium poisoning to prevent misdiagnosis. 2, chronic selenium poisoning: chronic selenium poisoning is often due to the daily intake of selenium from food 2400 ~ 3000 micrograms for several months before the symptoms appear. The symptoms are hair loss, nail loss, yellow skin, bad breath, fatigue, increased susceptibility to dental caries, depression, etc. Generally chronic selenium toxicity has dizziness, headache, tiredness and weakness, metallic taste in the mouth, nausea, vomiting, loss of appetite, diarrhea, garlic odor of breath and sweat, but also hepatomegaly, abnormal liver function, autonomic dysfunction, and increased urinary selenium. Long-term high selenium makes children with delayed growth in length and weight, rough and fragile hair, and even neurological symptoms and intelligent changes. The main features of chronic selenium toxicity are hair loss and nail shape changes, deformities and cirrhosis, and in some cases, skin lesions and neurological abnormalities can be seen.