Selenium is a very important trace element, which is of great importance for human functions, especially for the growth and development of children. In the human body, selenium is mainly involved in the formation of glutathione reductase, has direct and indirect antioxidant effects, can protect the function of cardiovascular and cerebrovascular and sensory system, stimulate the secretion of immunoglobulin, and relieve the toxic side effects of some heavy metals. However, the onset of selenium deficiency in human body is very insidious and does not show any specific symptoms, so it is difficult to be detected clinically. Especially in children, selenium deficiency can lead to growth retardation, decreased immunity and resistance, which has attracted more and more attention in recent years. Selenium deficiency needs to do what related tests: The manifestation of selenium deficiency is mainly hair loss, nail loss, some patients appear skin symptoms, a few patients can appear neurological symptoms and dental damage. People with mild or moderate selenium deficiency, the signs or symptoms are not obvious. It is best to do a trace element test to determine. The human body is made up of more than 60 elements. Depending on the amount of elements in the body, they can be divided into two categories: macronutrients and trace elements. Elements that account for more than 0.01% of the total weight of the body, such as calcium, phosphorus, magnesium, sodium, etc., are called macroelements; elements that account for less than 0.01% of the total weight of the body, such as iron, zinc, copper, iodine, selenium, manganese, etc., are called trace elements. Some macronutrients that are important to the body, such as calcium and magnesium, are also the subject of trace element testing for babies. Trace element testing is done by extracting the baby’s hair or blood and testing the amount of trace elements in it to determine if the baby has a trace element deficiency based on the criteria. 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 the human body 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 Creutzfeldt-Jakob 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 and nail loss. Serum selenium (Se): The total amount of selenium in the body of adults is about 14-21 mg. After the selenium absorbed from the intestine enters the blood plasma, it is mainly combined with globulin and lipoprotein in the blood and transported to the tissues of the whole body. Its main storage in the liver, kidney, spleen, lung, heart muscle, blood, of which about 1/3 of selenium exists in glutathione peroxidase, with antioxidant effect. Its content is rarely excreted before the body is saturated, and the excess selenium that enters the body can be converted to dimethyl selenium with volatility in the liver and excreted from the exhalation, and the main excretion pathway of selenium is still urine. The physiological functions of selenium in the body are to maintain normal myocardial metabolism; as a natural antioxidant and natural detoxifier of heavy metals in the body; selenium is also related to immunity, maintaining the physiological functions of heart, brain, liver, muscle and immunity. Chronic selenium deficiency in humans is also closely related to the occurrence of certain cancers, cardiovascular diseases, endemic cardiomyopathy (Keshan disease) and other diseases. Whole blood selenium: the total amount of selenium in the adult body is 14-21 mg. After the selenium absorbed from the intestine enters the blood plasma, it is mainly combined with globulins and lipoproteins in the blood and transported to the tissues of the whole body. It is mainly stored in liver, kidney, spleen, lung, heart muscle and blood, of which about 1/3 of selenium exists in glutathione peroxidase. Its content is rarely excreted before the body is saturated. Excess selenium entering the body can be converted in the liver to dimethyl selenium with volatility and excreted from the exhaled breath. The main excretion route of selenium remains the urine. The physiological functions of selenium in the body are to maintain normal myocardial metabolism, to act as a natural antioxidant in the body and a natural detoxifier of heavy metals, and selenium is also related to immunity. Chronic selenium deficiency in humans is also closely related to the occurrence of certain cancers, cardiovascular diseases, endemic cardiomyopathies (Keshan disease) and other diseases.