Low levels of 25-hydroxyvitamin D, the active form of vitamin D present in the body, are generally indicative of vitamin D deficiency. The causes of vitamin D deficiency are as follows: insufficient exposure to sunlight; history of vitamin D deficiency in the pregnant mother, insufficient vitamin D nutrition in the mother during the second trimester of pregnancy, preterm babies, multiple births, failure to take vitamin D supplements in a timely manner, failure to add vitamin D-rich foods such as egg yolks and liver in a timely manner; rapid growth rate of the child or suffering from gastrointestinal and hepatic-biliary disorders, chronic renal disease, long-term use of antiepileptic medications or drugs such as glucagon, which affects body metabolism and calcium and phosphorus absorption and utilization. Corticosteroids and other drugs, affecting the metabolism of vitamin D in the body and the absorption and utilization of calcium and phosphorus.