Synonyms: 1,25(OH)2D
Related tests: Calcium, PTH
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
Chronic kidney failure, rickets, vitamin D-resistant rickets, osteomalacia, hyperparathyroidism.
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
1,25(OH)2D does not reflect vitamin D stores, for which 25(OH)D is the preferred test.
1,25(OH)2D levels are affected by factors that the laboratory cannot control (season, UV exposure, ethnicity, dietary or drug intake, kidney function).
These factors must be taken into account by the requesting doctor.