1, vitamin D deficiency is mostly seen in vitamin D-dependent rickets caused by lack of vitamin D in food or too little exposure to sunlight. 2, chronic diarrhea, steatorrhea, obstructive jaundice can cause impaired absorption of vitamin D, which leads to reduced intestinal calcium absorption resulting in hypocalcemia. 3.In liver cirrhosis and renal failure, the body cannot generate active vitamin D3 due to impaired hydroxylation of vitamin D, resulting in anti-vitamin D rickets. 4, accelerated vitamin D catabolism: long-term application of antiepileptic drugs such as phenobarbital can shorten the half-life of vitamin D, which in turn causes hypocalcemia and calcium deficiency.