Vitamin D (1,25) Dihydroxy

This test measures the biologically active form of vitamin D, known as 1,25-dihydroxyvitamin D (calcitriol). Unlike the more commonly requested 25-hydroxyvitamin D, which reflects your overall vitamin D stores, this test assesses the amount of fully activated hormone available to your body. It is typically requested when a clinician needs to investigate abnormal calcium regulation, suspected kidney-related vitamin D activation problems, or rare conditions affecting phosphate and bone metabolism. It can also be useful in monitoring certain granulomatous or lymphoproliferative disorders where inappropriate activation of vitamin D occurs.

A simple blood sample is required, and the analysis is performed using a validated immunoassay or liquid chromatography-tandem mass spectrometry (LC-MS/MS) method, depending on the clinical requirement.

Vitamin D Metabolism
- 1,25-dihydroxyvitamin D (calcitriol): the active hormonal form of vitamin D, responsible for increasing intestinal calcium absorption, regulating bone mineralisation, and maintaining phosphate balance.

Why it matters
The active vitamin D hormone is central to calcium homeostasis and skeletal integrity. Measuring it directly helps distinguish between low vitamin D stores and impaired activation, which can occur in kidney disease or specific metabolic disorders. This distinction guides appropriate treatment and avoids unnecessary supplementation. It also supports the investigation of unexplained hypercalcaemia, hypocalcaemia, or abnormal bone turnover, giving you and your clinician a clearer picture of your calcium and bone health.