nutritional disorders
Vitamin D Deficiency and Dependency
Diagnosis ofVitamin DDeficiency and Dependency
Levels of 25(OH)D (D2 + D3) Vitamin Ddeficiency may be suspected based on any of the following: A history of inadequate sunlight exposure or dietary intake Symptoms and signs of rickets, osteomalacia, or neonatal tetany Characteristic bone changes seen on x-ray X-rays of the radius and ulna plus serum levels of calcium, phosphate, alkaline phosphatase,parathyroid hormone(PTH), and 25(OH)D are needed to differentiatevitamin Ddeficiency from other causes of bone demineralization. Assessment ofvitamin Dstatus and serologic tests for syphilis can be considered for infants with craniotabes based on the history and physical examination, but most cases of craniotabes resolve spontaneously. Rickets can be distinguished from chondrodystrophy because the latter is characterized by a large head, short extremities, thick bones, and normal serum calcium, phosphate, and alkaline phosphatase levels. Tetany due to infantile rickets may be clinically indistinguishable from seizures due to other causes. Blood tests and clinical history may help distinguish them. Bone changes, seen on x-rays, precede clinical signs. In rickets, changes are most evident at the lower ends of the radius and ulna. The diaphyseal ends lose their sharp, clear outline; they are cup-shaped and show a spotty or fringy rarefaction. Later, because the ends of the radius and ulna have become noncalcified and radiolucent, the distance between them and the metacarpal bones appears increased. The bone matrix elsewhere also becomes more radiolucent. Characteristic deformities result from the bones bending at the cartilage-shaft junction because the shaft is weak. As healing begins, a thin white line of calcification appears at the epiphysis, becoming denser and thicker as calcification proceeds. Later, the bone matrix becomes calcified and opacified at the subperiosteal level. In adults, bone demineralization, particularly in the spine, pelvis, and lower extremities, can be seen on x-rays; the fibrous lamellae can also be seen, and incomplete ribbonlike areas of demineralization (pseudofractures, Looser lines, Milkman syndrome) appear in the cortex. Because levels of serum 25(OH)D reflect body stores ofvitamin Dand correlate with symptoms and signs ofvitamin Ddeficiency better than levels of othervitamin Dmetabolites, the best way to diagnosevitamin Ddeficiency is generally considered to be 25(OH)D (D2 + D3) levels Target 25(OH)D levels are > 20 to 24 ng/mL (about 50 to 60 nmol/L) for maximal bone health; whether higher levels have other benefits remains uncertain, and higher absorption of calcium may increase risk of coronary artery disease. If the diagnosis is unclear, serum levels of 1,25-dihydroxyvitamin D and urinary calcium concentration can be measured. In severe deficiency, serum 1,25-dihydroxyvitamin D is abnormally low, usually undetectable. Urinary calcium is low in all forms of the deficiency except those associated with acidosis. Invitamin Ddeficiency, serum calcium may be low or, because of secondary hyperparathyroidism, may be normal. Serum phosphate usually decreases, and serum alkaline phosphatase usually increases. Serum PTH may be normal or elevated. Type I hereditaryvitamin D–dependent rickets results in normal serum 25(OH)D, low serum 1,25-dihydroxyvitamin D and calcium, and normal or low serum phosphate. 