Vitamin D deficiency refers to a condition characterized by low serum levels of vitamin D. Vitamin D is a fat-soluble vitamin that is responsible for calcium homeostasis and bone health. Vitamin D increases the efficiency of calcium and phosphorus absorption from the small intestine and aids in the maturation of osteoclasts in the bone.

Vitamin D deficiency results from either lack of exposure to sunlight (ultraviolet B) or impaired absorption. Malabsorption of vitamin D can occur in individuals with small bowel resections, celiac disease, inflammatory bowel disease (eg, Crohn disease, ulcerative colitis), cystic fibrosis, or protein-losing enteropathies. Chronic kidney disease may contribute to vitamin D deficiency by decreasing the conversion of 25-hydroxyvitamin D to the active form, 1,25-dihydroxyvitamin D. Some medications (eg, phenytoin) can impair vitamin D absorption as well. Exclusively breastfed infants are also at high risk for vitamin D deficiency without adequate supplementation, as are patients of any age with severe dietary restrictions, including low dairy intake. Patients who are institutionalized or chronically ill as well as older patients and individuals with dark skin color are also at high risk for vitamin D deficiency.

Patients with vitamin D deficiency are often asymptomatic, but children may present with bowing of the legs, a preference to sit, and rickets, while adults may present with chronic muscle aches and pains. In severe cases, adults may present with osteomalacia, a condition of defective bone mineralization.