25 OH Vitamin D

Vitamin D is converted into 25-OH vitamin D in the liver. This is the major storage form of vitamin D and is the analyte of choice for determination of the vitamin D status. Hypocalcaemia can result from vitamin D deficiency or disorders of vitamin D metabolism such as renal failure. Hyper and hypo-phosphataemia can each be caused by vitamin D excess and deficiency respectively.
Vitamin D deficiency prevents normal absorption of calcium from the gut leading to increased mobilisation of calcium from the bone in order to maintain blood concentrations. The resulting demineralisation of bone leads to bone weakness known as rickets in children, or osteomalacia in adults. Vitamin D deficiency can occur secondary to various diseases that cause fat malabsorption. These include cystic fibrosis, primary biliary cirrhosis, short bowel syndrome, pancreatic disease and Crohn’s disease. Vitamin D toxicity is due to over-consumption of supplements and cannot be caused by excess exposure to sunlight.