Vitamin D (25-OH Vitamin D)
Also known as: 25-hydroxyvitamin D, Calcidiol, 25(OH)D
Vitamin D deficiency affects an estimated 1 billion people worldwide and is particularly prevalent in the Middle East, where sun exposure is paradoxically limited by indoor lifestyles and clothing. This test measures your actual stored vitamin D level.
Venous blood (serum separator tube)
24–48 hours
Not required
Vitamins & Minerals
Reference Ranges
| Measurement | Reference Range | Unit |
|---|---|---|
| Deficient | ≤ 20 | ng/mL |
| Insufficient | 20 – 29 | ng/mL |
| Sufficient | 30 – 100 | ng/mL |
| Potential toxicity | ≥ 100 | ng/mL |
Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.
What This Test Measures
The 25-OH vitamin D test measures calcidiol — the storage form of vitamin D produced when the liver processes vitamin D from skin synthesis and dietary intake. It is distinct from the active form (1,25-OH vitamin D), which is tightly regulated and does not reflect overall vitamin D status. Calcidiol has a half-life of 15–25 days, making it a reliable measure of your vitamin D stores.
When Is This Test Ordered?
- Investigating fatigue, bone pain, or muscle weakness
- Monitoring supplementation in deficient patients
- Risk assessment in malabsorption syndromes (Crohn's, coeliac disease)
- Osteoporosis or fracture risk evaluation
- Routine screening in populations with high deficiency prevalence
When Results Are High
Vitamin D toxicity is rare from sun exposure alone — only from excessive supplementation. Above 100 ng/mL, hypercalcaemia becomes a risk (constipation, confusion, kidney stones). Most supplementation guidelines aim for 40–80 ng/mL.
When Results Are Low
Below 20 ng/mL impairs calcium absorption, weakens bones (osteomalacia in adults, rickets in children), and is associated with muscle weakness, fatigue, and immune dysfunction. Severe deficiency below 10 ng/mL significantly raises fracture risk.
Vitamin D3 vs D2 supplementation
Vitamin D3 (cholecalciferol) raises blood levels approximately 3 times more efficiently than D2 (ergocalciferol) per IU dose. Most guidelines now recommend D3 for supplementation. Daily dosing is preferred over weekly or monthly boluses for achieving stable levels.
In the Middle East and Gulf region, studies consistently show 60–80% of adults have levels below 20 ng/mL despite strong sunlight — driven by indoor work, vehicle commuting, and conservative clothing. The paradox of sun-rich regions with high deficiency rates is well-documented.
Common dosing guidelines
| Level (ng/mL) | Typical supplement dose | Retest timing |
|---|---|---|
| Below 10 (severe deficiency) | 50,000 IU weekly D3 × 8 weeks, then 2,000–4,000 IU/day | After loading course |
| 10–20 (deficiency) | 2,000–4,000 IU/day D3 | 3 months |
| 20–29 (insufficiency) | 1,000–2,000 IU/day D3 | 4–6 months |
| 30–100 (sufficient) | Maintenance 600–2,000 IU/day D3 | Annually |
Factors That Can Affect Your Result
- Skin pigmentation — darker skin requires more sun exposure for the same synthesis
- SPF 30+ sunscreen reduces vitamin D synthesis by up to 95%
- Obesity — vitamin D is fat-soluble and sequestered in adipose tissue
- Kidney disease impairs conversion of stored to active form
- Time of year — levels typically drop 20–30% from summer to winter in non-supplemented people
Tracking This Test Over Time
If supplementing, retest after 3 months to verify response. Not everyone absorbs oral vitamin D3 at the same rate — some people need higher doses or require the D3 taken with a fat-containing meal. Track your level until you find the dose that keeps you consistently above 30 ng/mL.
Track your Vitamin D in LoomsRelated Tests
Medical disclaimer
This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.