25-OH vitamin D
Bone metabolism is significantly influenced by the activity of osteoblasts and osteoclasts, the calciotropic hormones 1,25-dihydroxy vitamin D hormone), parathyroid hormone and calcitonin, and the extracellular calcium concentration.
Vitamin D are a group of structurally related hydrophobic vitamins, of which the isoforms vitamin D3 from animals and vitamin D2 from plants and fungi are the physiologically most important representatives. Vitamin D is hydroxylated in the liver to 25-hydroxy vitamin D (25-OH vitamin D) and in another hydroxylation step in the kidneys 1,25-dihydroxy vitamin D is produced. The vitamin D circulating in the blood is mainly bound to proteins. Up to 90% is present bound to the vitamin D binding protein, and approximately 10% bound to albumin. Less than 1% of the vitamin D is free. Free and albumin-bound vitamin D are considered as biologically available.
While 1,25-dihydroxy vitamin D is the biologically active form of vitamin D, 25-OH-Vitamin D makes up for the largest proportion of the vitamin D-related metabolites in blood and is thus the best indicator for the individual vitamin D status.
Numerous metabolic processes are regulated by vitamin D. Vitamin D plays an important part in the bone metabolism since it maintains the physiological calcium and phosphate levels in the serum and stimulates the resorption of calcium and phosphate in the intestine. Vitamin D deficiency therefore leads to reduced bone mineralisation, which manifests as rickets in children and as osteomalacia in adults.
Moreover, the combined determination of 25-OH vitamin D, parathyroid hormone and calcitonin levels may provide information on whether or not bone metabolism disorders or diseases are present.