Vitamin D (25-Hydroxyvitamin D)
A blood test measuring the body's vitamin D stores, essential for bone health, immune function, and calcium absorption.
Last updated: April 10, 2026
An estimated 42% of U.S. adults are vitamin D deficient. The 25-hydroxyvitamin D test is the most reliable way to check vitamin D status, and it is one of the most commonly ordered blood tests in routine health screenings.
Why Is Vitamin D Tested?
Vitamin D is a fat-soluble vitamin the body needs to absorb calcium and phosphorus, maintain strong bones, and support immune function. The body makes vitamin D when skin is exposed to sunlight, and it can also come from certain foods and supplements.
The 25-hydroxyvitamin D blood test is the standard way to measure vitamin D status. A healthcare provider may order this test to:
- Screen for vitamin D deficiency or insufficiency
- Monitor bone health, particularly in older adults
- Evaluate unexplained fatigue, muscle weakness, or bone pain
- Assess calcium metabolism or parathyroid disorders
- Monitor people with conditions that affect absorption (celiac disease, Crohn's disease, gastric bypass)
- Track supplement effectiveness over time
Vitamin D is often tested as part of routine blood work or alongside a comprehensive metabolic panel (CMP) when bone or calcium concerns are present.
Normal Reference Ranges
Reference ranges may vary by laboratory and testing method. Always use the ranges provided on the specific lab report.
| Status | Level (ng/mL) | Level (nmol/L) |
|---|---|---|
| Deficient | Less than 20 ng/mL | Less than 50 nmol/L |
| Insufficient | 20 – 29 ng/mL | 50 – 72 nmol/L |
| Sufficient (Normal) | 30 – 100 ng/mL | 75 – 250 nmol/L |
| Potentially Toxic | Greater than 100 ng/mL | Greater than 250 nmol/L |
| Unit | Optimal Range |
|---|---|
| ng/mL | 30 – 100 |
| nmol/L | 75 – 250 |
What Does a HIGH Vitamin D Level Mean?
Vitamin D levels above 100 ng/mL (250 nmol/L) are considered potentially toxic. This is uncommon and almost always caused by excessive supplement use rather than sun exposure or diet.
Possible causes of elevated vitamin D include:
- Supplement overuse — taking high-dose vitamin D supplements without medical guidance
- Granulomatous diseases — conditions like sarcoidosis that cause the body to produce excess active vitamin D
- Certain lymphomas — some cancers can increase vitamin D activation
Vitamin D toxicity can lead to hypercalcemia (dangerously high calcium), which may cause nausea, vomiting, weakness, frequent urination, kidney stones, and in severe cases, kidney damage or heart rhythm problems.
What Does a LOW Vitamin D Level Mean?
Low vitamin D is one of the most common nutritional deficiencies worldwide. Levels below 20 ng/mL are classified as deficient, while 20–29 ng/mL is considered insufficient.
Common causes of low vitamin D include:
- Limited sun exposure — spending most time indoors, living in northern latitudes, or wearing full-coverage clothing
- Darker skin pigmentation — melanin reduces the skin's ability to produce vitamin D from sunlight
- Dietary insufficiency — few foods naturally contain vitamin D
- Obesity — vitamin D can become sequestered in fat tissue, reducing circulating levels
- Malabsorption disorders — celiac disease, Crohn's disease, and gastric bypass surgery can impair absorption
- Liver or kidney disorders — these organs convert vitamin D to its active form
- Certain medications — anti-seizure drugs, glucocorticoids, and some antifungal medications
Over time, vitamin D deficiency may contribute to weakened bones (osteomalacia in adults, rickets in children), increased fracture risk, muscle weakness, chronic fatigue, and impaired immune function.
Factors That Can Affect Your Results
Several factors can influence vitamin D test results beyond actual vitamin D status:
- Season and geography — levels are typically higher in summer and lower in winter, especially at higher latitudes
- Time of day of blood draw — minor variations can occur, though this effect is small
- Recent supplement use — taking a large dose of vitamin D shortly before testing can spike results
- Body weight — higher body fat can lower circulating vitamin D levels
- Age — the skin's ability to produce vitamin D decreases with age
- Lab method — different assay types (immunoassay vs. LC-MS/MS) can produce slightly different values
- Pregnancy — vitamin D requirements increase during pregnancy
Should I Be Concerned?
A single vitamin D result outside the normal range does not necessarily indicate a serious problem. Mild insufficiency (20–29 ng/mL) is extremely common and often correctable with modest lifestyle or supplement changes.
Results that may warrant closer attention include:
- Levels below 12 ng/mL, which indicate severe deficiency
- Levels above 100 ng/mL, which suggest potential toxicity
- Persistently low levels despite supplementation, which may point to an absorption issue
- Abnormal results combined with symptoms like bone pain, fractures, or muscle weakness
A healthcare provider can interpret results in the context of overall health, symptoms, and other lab values like calcium and parathyroid hormone (PTH).
Common Next Steps
Depending on the result, a healthcare provider may recommend:
- Vitamin D supplementation — dosage varies based on the severity of deficiency and individual factors. A healthcare provider will determine the appropriate amount.
- Lifestyle modifications — increased sun exposure (10–15 minutes several times per week) and dietary changes (fatty fish, fortified dairy, egg yolks)
- Follow-up testing — rechecking levels after 2–3 months of supplementation
- Additional lab tests — calcium, phosphorus, parathyroid hormone (PTH), or a bone density scan (DEXA)
- Evaluation for underlying conditions — if deficiency persists despite supplementation, testing for malabsorption or kidney/liver disorders
Frequently Asked Questions
What is a normal vitamin D level?
A sufficient vitamin D level is generally 30–100 ng/mL (75–250 nmol/L). Levels of 20–29 ng/mL are considered insufficient, and levels below 20 ng/mL indicate deficiency, according to the Endocrine Society.
How common is vitamin D deficiency?
Very common. Research published in Nutrition Research found that approximately 42% of U.S. adults are vitamin D deficient. Risk factors include limited sun exposure, darker skin, obesity, living in northern latitudes, and being over age 65.
What is the difference between vitamin D2 and D3?
Vitamin D2 (ergocalciferol) comes from plant sources and fortified foods. Vitamin D3 (cholecalciferol) is produced by the skin in response to sunlight and found in animal-based foods. Research suggests D3 is more effective at raising blood levels. The 25-hydroxyvitamin D test measures the combined total of both forms.
Can too much vitamin D be dangerous?
Yes. Levels above 100 ng/mL can lead to vitamin D toxicity, causing hypercalcemia (excess calcium in the blood). Symptoms include nausea, vomiting, weakness, and kidney damage. Toxicity almost always results from excessive supplement use, not from sun exposure or diet.
Does sunscreen block vitamin D production?
Sunscreen can reduce vitamin D production in the skin, but in practice, most people do not apply it thickly or consistently enough to completely block it. Brief sun exposure (10–15 minutes) before applying sunscreen may help, though this varies by skin type and geographic location.
How long does it take to correct a deficiency?
With appropriate supplementation, most people see significant improvement within 2–3 months. A healthcare provider will typically recheck levels after this period and adjust dosing as needed. Severe deficiency may require higher-dose "loading" regimens.
Is vitamin D the same as a calcium test?
No. The vitamin D test measures 25-hydroxyvitamin D in the blood, while a calcium test measures calcium levels. However, because vitamin D is essential for calcium absorption, these tests are often ordered together when evaluating bone health.
Who is most at risk for vitamin D deficiency?
Higher-risk groups include older adults (reduced skin synthesis), people with darker skin (melanin reduces UV absorption), those who are obese, individuals with malabsorption disorders (celiac disease, Crohn's disease), people in northern latitudes, and those who rarely go outdoors.
Do vitamin D levels change with the seasons?
Yes. Vitamin D levels tend to be highest in late summer and lowest in late winter. This seasonal variation is more pronounced at higher latitudes, where UV-B radiation from sunlight is minimal during winter months. Testing in late winter may show lower levels than a mid-summer test in the same person.
What foods are high in vitamin D?
The best dietary sources of vitamin D include fatty fish (salmon, mackerel, sardines), cod liver oil, egg yolks, and fortified foods such as milk, orange juice, and cereals. However, it is difficult to get enough vitamin D from food alone, which is why sunlight and supplements play a significant role.
Sources
- Holick MF, et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2011;96(7):1911-1930.
- Forrest KYZ, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutrition Research. 2011;31(1):48-54.
- MedlinePlus. Vitamin D Test. U.S. National Library of Medicine.
- Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press; 2011.
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Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges vary by laboratory and individual factors. Always discuss results with a qualified healthcare provider.