Vitamin d

a prohormone that undergoes two sequential hydroxylations to yield the active hormone. The primary functions of vitamin D are the homeostatic maintenance of serum calcium and phosphorus levels and the mineralization of bone. Deficiency of vitamin D leads to rickets or osteomalacia, and excess can produce hypercalcaemia, mobilization of calcium from bone and kidney dysfunction.

In bones the active vitamin mobilizes calcium and phosphorus from previously formed bone by stimulating osteocytic osteolysis; it also promotes maturation and mineralization of organic matrix.

Vitamin D production is regulated by calcium, parathyroid hormone and phosphate levels, but it is also a self-regulator in that it affects its own production by both direct and indirect means.

Hypervitaminosis D can be acute or chronic. Manifestations of acute poisoning include vomiting, fever, dehydration, abdominal cramps, bone pain and tenderness, convulsions and coma. After chronic poisoning, lassitude, thirst, anorexia and polyuria are followed by vomiting, abdominal pain and diarrhoea. Albuminuria, haematuria, hypercalciuria and hypercalcaemia are found on laboratory studies.

Radiographs of the bones of infants and children show metaphyseal bands of increased density, reflecting heavy calcifications of the matrix of the proliferating cartilage, alternating with areas of increased lucency in the tubular bones. Both osteoporosis and cortical thickening may be observed, at different sites. Widespread osteosclerosis has also been noted. Other manifestations that may be present in some patients are metastatic calcification of viscera, blood vessels, periarticular structures, muscles, laryngeal and tracheal cartilage, the falx cerebri and the tentorium cerebelli.

In adults, hypervitaminosis D can lead to focal or generalized osteoporosis, especially in the bones of the appendicular skeleton, the spine, the pelvis and the skull. Massive soft tissue calcification can become apparent in certain sites, such as periarticular regions, bursae, tendon sheaths, joint capsules and intra-articular cavities. Metastatic soft tissue calcification is known to accompany hypervitaminosis D.

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