25-Hydroxyvitamin D
Overview
25-Hydroxyvitamin D [25(OH)D] is the major circulating form of vitamin D and serves as the best indicator of vitamin D nutritional status and metabolic function. It is formed in the liver by hydroxylation of vitamin D2 or D3 and reflects contributions from sunlight exposure, diet, and supplements before conversion to active 1,25-dihydroxyvitamin D in the kidney. Low levels indicate vitamin D deficiency associated with osteoporosis, rickets, osteomalacia, and increased fracture risk, while high levels suggest toxicity from excessive supplementation. Clinically, 25(OH)D measurement guides supplementation, assesses deficiency risk in at-risk populations, and monitors therapy response.
Clinical Use Cases
- Diagnosis of vitamin D deficiency or insufficiency.
- Assessment of osteoporosis and fracture risk.
- Evaluation of rickets/osteomalacia in children and adults.
- Monitoring response to vitamin D supplementation.
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Screening high-risk groups (elderly, malabsorption, obesity, dark skin).
Specimen Types
- Serum.
- Plasma (EDTA or heparin).
Measurement Methods
- Liquid chromatography-tandem mass spectrometry (LC-MS/MS).
- Automated immunoassays (chemiluminescent, electrochemiluminescent).
- Radioimmunoassay (less common).
Test Preparation and Influencing Factors
- No fasting required.
- Recent vitamin D supplementation affects levels within 2-4 weeks.
- Obesity sequesters vitamin D in adipose tissue, lowering circulating levels.
- Malabsorption syndromes (celiac, IBD, bariatric surgery) reduce levels.
- Seasonal variation and latitude influence baseline levels.
- Liver disease impairs 25-hydroxylation.
Synonyms
- 25(OH)D.
- 25-hydroxyvitamin D.
- Calcidiol.