Vitamin D (25-OH)
25(OH)D· also: 25-OH D, Vitamin D, 25-hydroxyvitamin D
Clinical Overview
25-hydroxy Vitamin D (calcidiol) is the storage form of vitamin D and the best test to measure overall vitamin D status. It reflects vitamin D obtained from sunlight exposure, diet, and supplements. Vitamin D is essential for calcium absorption, bone mineralization, immune function, and muscle strength.
Why This Test Matters
Vitamin D deficiency is a global pandemic affecting over 1 billion people. It is the direct cause of rickets in children and osteomalacia in adults, and a contributing factor to osteoporosis. Deficiency is linked to increased risk of infections, autoimmune diseases, type 2 diabetes, cardiovascular disease, depression, and several cancers — though causality for these associations is still under investigation. All patients with osteoporosis, chronic kidney disease, malabsorption, or taking corticosteroids should be screened.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 30 – 100 | ng/mL | Sufficient |
| 20 – 29 | ng/mL | Insufficient |
Also reported in: nmol/L.Conversion factor: 1 nmol/L = 2.500 ng/mL.
Critical (Panic) Values
Critical Low: < 10 ng/mL. Critical High: > 150 ng/mL. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High 25(OH)D Causes
- Vitamin D toxicity from excessive supplementation (most common)
- Granulomatous diseases (sarcoidosis, TB, histoplasmosis — produce active vitamin D)
- Williams syndrome (genetic hypersensitivity)
Low 25(OH)D Causes
- Insufficient sun exposure (indoor lifestyle, dark skin, living at high latitudes)
- Dietary deficiency (vegans, low dairy intake)
- Malabsorption syndromes (celiac disease, Crohn's disease, gastric bypass)
- Chronic kidney disease (impaired vitamin D activation)
- Liver disease (impaired 25-hydroxylation)
- Obesity (vitamin D sequestered in fat tissue)
- Medications: anticonvulsants (phenytoin, carbamazepine), rifampicin, glucocorticoids
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting required. For accurate results, no vitamin D supplements for 24 hours before testing (although most labs measure steady-state 25-OH D, which changes slowly). Note the time of year — levels are naturally lower in winter.
Factors That Can Affect Results
- Season (significantly lower in winter at high latitudes)
- Skin color (melanin reduces UV-induced vitamin D synthesis)
- Body weight (obesity sequesters vitamin D, lowering levels)
- Sunscreen (blocks UV-B synthesis if applied consistently)
- Sample handling (protect from UV light)
Related Topics
Frequently Asked Questions
What vitamin D level is considered deficient?
Vitamin D below 20 ng/mL (50 nmol/L) is considered deficient by most guidelines. Levels 20–29 ng/mL are insufficient. Levels 30–100 ng/mL are sufficient. Above 100 ng/mL can indicate toxicity risk. However, optimal levels are debated — some experts recommend maintaining levels above 40–60 ng/mL for immune and musculoskeletal benefits, while others argue benefits plateau above 30 ng/mL.
How much vitamin D supplement should I take?
The safe upper limit for adults is 4,000 IU/day, though short-term doses of 5,000–10,000 IU/day are used under medical supervision to correct deficiency. The recommended dietary allowance (RDA) is 600–800 IU/day for most adults, rising to 800–1,000 IU/day for adults over 70. However, to raise a deficient level efficiently, 2,000–4,000 IU/day is typically needed. Always aim to correct to the sufficient range (30–100 ng/mL) based on follow-up testing.
Does high vitamin D prevent cancer?
The relationship between vitamin D and cancer is complex. Observational studies show associations between low vitamin D and higher risks of colorectal, breast, and prostate cancers. However, large randomized trials (VITAL trial) showed that vitamin D supplementation did not significantly reduce cancer incidence. There was a suggestion of reduced cancer mortality with supplementation in those with a normal BMI. At this time, cancer prevention is not a validated indication for vitamin D supplementation beyond correcting deficiency.