Unit Converter
Vitamin D total
(25-OH Vitamin D - Gold Standard Marker of Vitamin D Status)
Synonyms
- 25-hydroxyvitamin D
- 25-OH D
- Calcidiol
- Vitamin D (total)
- 25(OH)D₂ + 25(OH)D₃
- Total Vitamin D
- Calcitriol precursor
Units of Measurement
- nmol/L
- ng/mL
- ng/dL
- ng/100 mL
- ng%
- ng/L
- µg/L
Unit Conversions
Molecular weight of 25-hydroxyvitamin D ≈ 400.65 g/mol
nmol/L ↔ ng/mL
1 ng/mL=2.5 nmol/L1\ \text{ng/mL} = 2.5\ \text{nmol/L}1 ng/mL=2.5 nmol/L 1 nmol/L=0.4 ng/mL1\ \text{nmol/L} = 0.4\ \text{ng/mL}1 nmol/L=0.4 ng/mL
ng/dL → ng/mL
1 ng/dL=0.01 ng/mL1\ \text{ng/dL} = 0.01\ \text{ng/mL}1 ng/dL=0.01 ng/mL
ng/mL → ng/L
1 ng/mL=1000 ng/L=1 µg/L1\ \text{ng/mL} = 1000\ \text{ng/L} = 1\ \text{µg/L}1 ng/mL=1000 ng/L=1 µg/L
ng/100 mL = ng% = ng/dL
Description
25-hydroxyvitamin D (25-OH D) is the primary circulating form and the best indicator of overall vitamin D stores.
It reflects:
- Cutaneous synthesis (UVB)
- Dietary intake
- Supplementation
Total Vitamin D = 25-OH D₂ + 25-OH D₃
It is hydroxylated in:
- Liver → 25-OH D (calcidiol)
- Kidney → 1,25-OH₂ D (calcitriol: active form)
25-OH D has a long half-life (2–3 weeks), making it ideal for screening.
Physiological Role
1. Calcium / Phosphate Homeostasis
- Enhances intestinal calcium absorption
- Improves phosphate absorption
- Regulates PTH levels
2. Bone Health
Essential for:
- Prevention of rickets
- Prevention of osteomalacia
- Bone remodeling
3. Immune Function
Vitamin D modulates innate and adaptive immunity.
4. Endocrine / Metabolic Functions
Low vitamin D links with:
- Insulin resistance
- Autoimmune disease
- Muscle weakness
Clinical Significance
VITAMIN D DEFICIENCY
Symptoms
- Bone pain
- Muscle weakness / myopathy
- Fatigue
- Increased falls in elderly
- Osteomalacia (adults)
- Rickets (children)
Causes
- Low sunlight exposure
- Dark skin
- Obesity
- Malabsorption (celiac, Crohn’s, post-bariatric surgery)
- Liver disease
- Kidney disease
- Anticonvulsants, rifampicin, steroids
EXCESS VITAMIN D
Usually due to mega-dose supplements, not sunlight.
Symptoms
- Hypercalcemia
- Nausea, vomiting
- Polyuria, dehydration
- Kidney stones
- Arrhythmias
Reference Intervals
(Endocrine Society + Tietz 8E + Mayo + ARUP)
Serum 25-OH Vitamin D
| Level | ng/mL | nmol/L |
| Severe deficiency | <10 | <25 |
| Deficiency | <20 | <50 |
| Insufficiency | 20–30 | 50–75 |
| Sufficiency (adequate) | 30–100 | 75–250 |
| Potential toxicity | >150 | >375 |
Diagnostic Uses
1. Evaluation of Vitamin D Deficiency
Most common indication.
2. Bone Metabolic Disorders
- Osteoporosis
- Osteomalacia
- Rickets
3. Chronic Kidney Disease
25-OH D used before calcitriol replacement.
4. Hyperparathyroidism
Low vitamin D causes secondary hyperparathyroidism.
5. Malabsorption Syndromes
Celiac, pancreatitis, bariatric surgery.
6. High-Risk Pregnancy
Vitamin D deficiency is common in pregnancy.
Analytical Notes
- LC-MS/MS is the gold standard.
- Immunoassays may underestimate in deficiency or overestimate with supplements.
- Fasting not required but recommended for consistency.
- Avoid light exposure (25-OH D is stable but precautions preferred).
Clinical Pearls
- Obesity reduces circulating vitamin D (fat sequestration).
- CKD patients require both 25-OH D and 1,25-OH₂ D evaluation.
- Vitamin D₃ supplements raise levels more effectively than Vitamin D₂.
- Daily dosing is safer than large bolus doses.
- Low vitamin D commonly coexists with hypomagnesemia, reducing response to therapy.
Interesting Fact
Vitamin D is the only vitamin that acts as a pro-hormone, with its active form functioning like a steroid hormone regulating gene transcription.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Vitamin D
- Endocrine Society Clinical Practice Guideline - Vitamin D, 2024 update
- Institute of Medicine (IOM/NAM) Vitamin D Requirements
- Mayo Clinic Laboratories - 25-OH Vitamin D
- ARUP Consult - Vitamin D Testing
- NIH Office of Dietary Supplements - Vitamin D