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Vitamin D, 25-Hydroxyvitamin D (Calcidiol)
(Primary Circulating Form of Vitamin D - Most Accurate Marker of Vitamin D Stores)
Synonyms
- 25-Hydroxyvitamin D
- 25-OH-D
- Calcidiol
- 25-hydroxycholecalciferol
- 25(OH)D₂ + 25(OH)D₃ (Total)
- Vitamin D status marker
- Pre-hormone form of Vitamin D
Units of Measurement
- nmol/L
- µg/L
- µg/dL
- µg/100 mL
- µg%
- ng/mL
Description
25-Hydroxyvitamin D (Calcidiol) is the major circulating and most reliable indicator of vitamin D status.
It reflects:
- UVB-driven skin synthesis
- Dietary intake
- Supplementation
- Long-term body stores
Why it is measured:
Because 25-OH D has a half-life of 2–3 weeks and represents total body vitamin D supply before conversion to the active hormone.
Conversion pathway:
- Skin → Vitamin D₃ (cholecalciferol)
- Liver → 25-OH D (Calcidiol) (measured analyte)
- Kidney → 1,25-OH₂ D (Calcitriol) - active hormone
25-OH D is preferred for testing.
1,25-OH₂ D is NOT a status test and may be normal even when 25-OH D is low.
Physiological Role of 25-OH D
Although inactive, calcidiol is crucial because it determines availability for conversion to the active hormone.
1. Calcium & Phosphate Homeostasis
Provides substrate for calcitriol production.
2. Bone Health
- Prevents rickets (children)
- Prevents osteomalacia (adults)
- Supports bone mineral density
3. Muscle Function
Vitamin D deficiency → myopathy, increased falls.
4. Immune Modulation
Vitamin D receptors present on immune cells.
5. Hormonal, Metabolic & Autoimmune Roles
Low levels linked to insulin resistance and immune dysregulation.
Clinical Significance
LOW 25-OH D
Extremely common.
Symptoms
- Bone pain
- Muscle weakness
- Fatigue
- Increased fractures
- Growth failure (children)
- Rickets
- Osteomalacia
Causes
- Low sunlight exposure
- Dark skin
- Obesity (adipose sequestration)
- Malabsorption (celiac, Crohn’s, pancreatitis)
- Liver disease
- Kidney disease
- Anticonvulsants, rifampicin, glucocorticoids
- Elderly, institutionalized
- Post-bariatric surgery
HIGH 25-OH D
Usually due to over-supplementation, not sunlight.
Symptoms
- Hypercalcemia
- Polyuria, dehydration
- Nausea, vomiting
- Nephrocalcinosis
- Kidney stones
- Confusion
Toxicity seen when levels >150 ng/mL (375 nmol/L).
Reference Intervals
(Endocrine Society 2024 + Tietz 8E + Mayo + ARUP)
25-OH Vitamin D
| Category | ng/mL | nmol/L |
| Severe Deficiency | <10 | <25 |
| Deficiency | <20 | <50 |
| Insufficiency | 20–30 | 50–75 |
| Sufficiency | 30–100 | 75–250 |
| Excess / Risk Toxicity | >100 | >250 |
| Toxicity | >150 | >375 |
Diagnostic Uses
1. Assessment of Vitamin D Deficiency
Primary test for:
- Osteoporosis
- Rickets
- Osteomalacia
- Secondary hyperparathyroidism
2. CKD Evaluation
Assesses substrate availability before giving calcitriol.
3. Malabsorption Detection
GI disorders, bariatric surgery.
4. High-Risk Pregnancy Monitoring
5. Autoimmune and Metabolic Disorders
Analytical Notes
- LC-MS/MS is the gold standard; distinguishes D₂ and D₃.
- Immunoassays can cross-react or underestimate levels.
- Fasting preferred but not mandatory.
- Samples stable but should be protected from light.
Clinical Pearls
- Obesity strongly lowers 25-OH D due to sequestration in fat.
- Vitamin D₂ supplementation raises D₂ but may not fully normalize total D.
- CKD patients may have normal 25-OH D yet low 1,25-OH₂ D.
- Daily dosing safer and more stable than large intermittent boluses.
- Magnesium deficiency prevents correction of vitamin D deficiency.
Interesting Fact
25-OH Vitamin D circulates bound to Vitamin D–binding protein (DBP), which maintains a large reserve - hence deficiency takes weeks to reverse.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Vitamin D
- Endocrine Society Clinical Practice Guideline, 2024 - Vitamin D in Adults
- Institute of Medicine (IOM/NAM) Dietary Reference Intakes for Vitamin D
- Mayo Clinic Laboratories - 25-Hydroxyvitamin D
- ARUP Consult - Vitamin D Testing
- NIH Office of Dietary Supplements - Vitamin D