The Vitamin D 25-Dihydroxy test measures the active form of Vitamin D which is produced in the liver and kidneys through the conversion of Vitamin D 25-Hydroxy. This test is not typically used as a routine measurement to assess if a person has a vitamin D deficiency because it may show normal results even if a person has an overall deficiency.
This test measures the bioactive form of vitamin D. It is used in the differential diagnosis of hypocalcemia and to monitor patients with renal osteodystrophy or chronic renal failure. This test is not suitable for diagnosis of vitamin D deficiency and monitoring supplementation in most patients. The Vitamin D 25-Hydroxy Blood Test is the recommended test for those purposes.
In some cases, the Vitamin D 1,25 Dihydroxy Blood Test is ordered if an abnormality associated with the enzyme that converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D or kidney disease is suspected. Low levels may be indicative of the latter, as this is often one of the first changes to take place in patients with early kidney failure.
This test may also be ordered to aid in the diagnosis or Parathyroid disorder or kidney failure. It also aids in the diagnosis of primary hyperparathyroidism, hypoparathyroidism, pseudohypoparathyroidism, renal osteodystrophy, and vitamin D-resistant rickets.
Calcitriol is a form of vitamin D that is used to treat and prevent low levels of calcium in the blood of patients whose kidneys or parathyroid glands (glands in the neck that release natural substances to control the amount of calcium in the blood) are not working normally. Low blood levels of calcium may cause bone disease.
A low level of 1,25-dihydroxyvitamin D can be seen in kidney disease and is one of the earliest changes to occur in persons with early kidney failure.
A high level of 1,25-dihydroxyvitamin D may occur when there is excess parathryoid hormone or when there are diseases, such as sarcoidosis or some lymphomas, that can make 1,25-dihydroxyvitamin D outside of the kidneys.