Yes, 25(OH)D and Vitamin D 25-Hydroxy Are the Same Thing
25(OH)D and vitamin D 25-hydroxy are identical terms referring to 25-hydroxyvitamin D, the primary circulating form of vitamin D and the standard biomarker used to assess vitamin D status. 1
Understanding the Terminology
What These Terms Mean
- 25(OH)D is simply the abbreviated scientific notation for 25-hydroxyvitamin D 1
- Vitamin D 25-hydroxy is the descriptive name for the same molecule 1
- Both terms refer to the metabolite formed when vitamin D (either D2 or D3) undergoes hydroxylation in the liver at the 25-carbon position 2, 3
Why This Is the Standard Test
- Serum 25(OH)D is recognized as the best indicator of vitamin D status because it reflects total body vitamin D stores from both dietary intake and skin synthesis 1, 2
- This metabolite has a half-life of approximately 2-3 weeks, making it stable enough for accurate measurement 1
- The measurement should include both 25(OH)D2 (from ergocalciferol) and 25(OH)D3 (from cholecalciferol) to capture total vitamin D status 1, 4
Important Clinical Distinctions
What 25(OH)D Is NOT
- 25(OH)D is NOT the same as 1,25-dihydroxyvitamin D [1,25(OH)2D], which is the biologically active form of vitamin D 2, 3
- Measuring 1,25(OH)2D provides no useful information about vitamin D status and is often normal or even elevated in vitamin D deficiency due to secondary hyperparathyroidism 2
- 25(OH)D is NOT the same as vitamin D3 (cholecalciferol) or vitamin D2 (ergocalciferol), which are the parent compounds before liver metabolism 2, 3
Measurement Considerations
- The assay used should measure total 25(OH)D, which is the sum of both 25(OH)D2 and 25(OH)D3 1, 4
- LC-MS/MS (liquid chromatography-mass spectrometry) has become the gold standard methodology for measuring 25(OH)D 1
- Variability between different assay methods can range from 10-20%, and classification of samples as "deficient" versus "nondeficient" can vary by 4-32% depending on which assay is used 4
Clinical Reference Ranges
Target Levels
- A minimum level of 30 ng/mL (75 nmol/L) is recommended for optimal health benefits, particularly for anti-fracture efficacy 1, 5
- Levels between 20-30 ng/mL represent insufficiency 1, 6
- Levels below 20 ng/mL represent deficiency requiring treatment 1, 6
- The upper safety limit is generally set at 100 ng/mL 1, 5