Mild-to-modest deficiency can be associated with osteoporosis or secondary hyperparathyroidism.

The exact levels of optimal circulating 25HDN concentrations remain a matter of debate. Inactivation of 25HDN and DHVD by CYP24A1 is a crucial process that prevents over production of DHVD and resultant vitamin D toxicity.īased on these considerations circulating 25HDN is the best indicator of optimal vitamin D body stores. This process, regulated by PTH, might increase DHVD synthesis at the expense of the alternative hydroxylation ( CYP24A1 ) product 24,25D. Like other steroid hormones, DHVD binds to a nuclear receptor, influencing gene transcription patterns in target organs.Ģ5HDN may also be converted into the inactive metabolite 24,25-dihydroxyvitamin D (24,25D) by ( CYP24A1 ) hydroxylation.

1-Alpha-hydroxylation ( CYP27B1) occurs on demand, primarily in the kidneys, under the control of parathyroid hormone (PTH) before expressing biological activity. Biological activity is expressed in the form of DHVD, the active metabolite of 25HDN. 25HDN is a prohormone that represents the main reservoir and transport form of vitamin D, being stored in adipose tissue and tightly bound by a transport protein while in circulation. Vitamin D may also be endogenously derived by conversion of 7-dihydrocholesterol to 25-hydroxyvitamin D3 in the skin upon ultraviolet exposure.Ģ5HDN is subsequently formed by hydroxylation ( CYP2R1) in the liver. Vitamin D compounds in the body are exogenously derived by dietary means from plants as 25-hydroxyvitamin D2 (ergocalciferol or calciferol) or from animal products as 25-hydroxyvitamin D3 (cholecalciferol or calcidiol). DHVD testing alone may not clearly indicate deficiencies of vitamin D stores. In the presence of renal disease, testing 1,25-dihydroxyvitamin D (DHVD) levels might be needed to adequately assess vitamin D status. This test is the preferred initial test for assessing vitamin D status and most accurately reflects the body's vitamin D stores.

Vitamin D is a generic designation for a group of fat-soluble, structurally similar sterols, which act as hormones.

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