Related tests: Calcium, parathyroid hormone, Alkaline phosphatase
Clinical significance
Vitamin D is a fat-soluble vitamin involved in calcium-phosphorus metabolism and exists in two forms: cholecalciferol (vitamin D3), which is made in the skin from 7-dehydrocholesterol through ultraviolet irradiation but can also be obtained from animal fats (fish liver, milk, meat); and ergocalciferol (vitamin D2), which is formed through ultraviolet irradiation from ergosterol taken in with the diet (yeast).
Both forms undergo two important hydroxylations in the body: the first in the liver, forming calcidiol (25(OH)D); the second in the kidney, forming calcitriol (1,25(OH)2D), which is the biologically active metabolite. Vitamin D acts mainly by increasing intestinal calcium absorption and on bone resorption (at low doses it promotes mineralisation; at high doses it mobilises calcium by stimulating osteoclasts), while also increasing tubular reabsorption of phosphorus in the kidney.
Clinical indications
25(OH)D deficiency is seen in childhood rickets, osteoporosis, osteomalacia, pregnancy, during growth, in older people, at the menopause, in malabsorption and in liver failure.
Sample type
The patient must have a blood sample taken.
Preparation
You must fast for at least 8 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes.
Notes
Aluminium hydroxide, colestyramine, corticosteroids, anticonvulsants, isoniazid and rifampicin lower its concentration.
Seasonal variation is due to differences in exposure to sunlight, which increases the body’s own production of cholecalciferol. 25(OH)D values below 10 ng/ml indicate significant deficiency. Vitamin D toxicity may occur with 25(OH)D values above 100 ng/ml.