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Measurement of 1,25(OH)2 Vitamin D

Vitamin D is mainly synthesized in the skin from 7-dehydrocholesterol and is partially from dietary origin. In the liver, Vitamin D is hydroxylated on carbon 25 to produce the obligatory intermediate 25OH Vitamin D. 25OH Vitamin D must be metabolized further before it can carry out the functions of Vitamin D on intestine, kidney and bone. This subsequent reaction takes place in the kidney and in other tissues. Thus 25OH Vitamin D is further hydroxylated in the 1st-position to produce 1,25(OH)2 Vitamin D.

1,25(OH)2 Vitamin D is the active form of Vitamin D with regard to the known functions whereas 25OH Vitamin D and Vitamin D itself can be excluded as being physiologically functional. 1,25(OH)2 Vitamin D stimulates the intestinal absorption of both calcium and phosphorus. It also stimulates bone resorption and mineralization thereby preventing the development of rickets and osteomalacia.

The levels of 1,25(OH)2 Vitamin D in plasma or serum is 100 to 1000 less than that of 25OH Vitamin D. Due to its low concentrations and the presence of many similar metabolites, the measurement of 1,25(OH)2 Vitamin D requires extraction and separation either by HPLC or by column chromatography preferable with the use of organic solvent. This extraction method is still recognized as the Gold Standard for 1,25(OH)2 Vitamin D testing.


The measurement of circulating 1,25(OH)2 Vitamin D is indicated in several disorders affecting calcium metabolism such as:

  • Assessment of Kidney functions: Chronic kidney failure and Haemodialysis (after kidney transplants)
  • Hyper and hypo-parathyroidism,
  • Rickets disease
  • Tumor-associated hypercalcemia
  • Osteomalacia as a results of various types of Vit D metabolism disturbances


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