Vitamin D is obtained from food and supplements or made in the skin by exposure to sunlight. 25-OH Vitamin D is formed from these Vitamin D sources and stored in the liver. 25-OH Vitamin D is then changed to its active form, 1,25-dihydroxy Vitamin D, in the kidney. 1,25-dihydroxy Vitamin D keeps normal levels of calcium and phosphate in the blood through its actions on the intestines, kidneys, and bones.
The most reliable clinical indicator of Vitamin D status is 25-OH Vitamin D because serum or plasma levels reflect the body’s storage levels of Vitamin D. 25-OH Vitamin D levels correlate with the clinical symptoms of Vitamin D deficiency.
Abnormal 1,25-dihydroxy Vitamin D levels may be seen in primary hyperparathyroidism, physiologic hyperparathyroidism secondary to low calcium or vitamin D intake, hypoparathyroidism, and chronic renal failure. 1,25-dihydroxy Vitamin D levels may be needed to adequately assess Vitamin D status in the presence of renal disease.
25-OH Vitamin D level is performed by PHL and should be ordered as test code VD251. 1,25-dihydroxy Vitamin D is performed by Quest Diagnostics and should be ordered as test code VITD2.