Vitamin D (25-Hydroxy) Blood Test: Deficiency, Insufficiency and Toxicity Explained
25-hydroxy vitamin D is the correct test for status — not 1,25-dihydroxy. Learn the deficiency cut-off, when to supplement, and when a level is too high.
25-hydroxy vitamin D is the storage form and the correct test for vitamin D status. Deficiency is < 30 nmol/L (< 12 ng/mL); insufficiency 30–50 nmol/L; sufficiency 50–125 nmol/L (20–50 ng/mL). Never order 1,25-dihydroxy vitamin D for routine screening — it is only useful in specific renal/parathyroid disease.
Vitamin D deficiency remains the most over-tested and over-treated micronutrient in primary care. The 2011 IOM guidance and 2024 Endocrine Society statement agree that 50 nmol/L (20 ng/mL) is sufficient for musculoskeletal health in the general population. Screening asymptomatic adults is not recommended by the USPSTF (Grade I — insufficient evidence). Targeted testing is appropriate for osteoporosis, chronic kidney disease, malabsorption (coeliac, IBD, bariatric surgery), dark skin at high latitudes, and persistent musculoskeletal pain. Supplement 800–2000 IU/day for most deficient adults; higher doses (up to 4000 IU/day) are safe. Toxicity is rare below 250 nmol/L. Vitamin D supplementation does NOT reduce cardiovascular events, cancer, falls (in non-deficient adults), or COVID mortality in high-quality RCTs (VITAL, D-Health trials).
- Correct test
- 25-OH-D (not 1,25-di-OH)
- Deficiency
- < 30 nmol/L (< 12 ng/mL)
- Sufficient
- 50–125 nmol/L (20–50 ng/mL)
- Typical dose
- 800–2000 IU/day
- Toxicity
- > 250 nmol/L (> 100 ng/mL)
Vitamin D status categories
| Level (nmol/L) | Level (ng/mL) | Status | Action |
|---|---|---|---|
| < 30 | < 12 | Deficient | Treat: 50,000 IU/week × 8 weeks then maintain |
| 30 – 50 | 12 – 20 | Insufficient | Supplement 1000–2000 IU/day |
| 50 – 125 | 20 – 50 | Sufficient | No action |
| 125 – 250 | 50 – 100 | High-normal | Reduce supplement |
| > 250 | > 100 | Toxic risk | Stop; check calcium |
Who should be tested
Symptoms of deficiency: bone pain, proximal muscle weakness, fatigue, waddling gait, fragility fracture.
Risk groups: dark skin at high latitude, veiled or housebound, malabsorption (coeliac, IBD, post-bariatric), chronic kidney disease stage 3+, osteoporosis, chronic corticosteroid use, obesity.
Do NOT screen asymptomatic low-risk adults — USPSTF grade I.
- Vitamin D < 30 nmol/L PLUS low corrected calcium (< 2.20 mmol/L) — treat rapidly.
- Symptomatic hypercalcaemia on vitamin D supplements — stop and check calcium urgently.
- Persistent bone pain despite replacement — investigate osteomalacia with ALP, phosphate, PTH.
Low vitamin D workup
- 125-OH-D < 30 nmol/L?Loading dose 50,000 IU weekly × 6–8 weeks, then 800–2000 IU daily maintenance.
- 2Not responding to supplement?Check adherence, then investigate malabsorption (tTG-IgA, faecal calprotectin).
- 3Persistent deficiency + high PTH?Check renal function, magnesium; endocrine referral.
Related questions people ask
- What is 1,25-dihydroxy vitamin D and when is it useful?
- Should I fast for vitamin D testing?
- Does vitamin D prevent falls?
- How does magnesium relate to vitamin D?
- Ferritin and iron status connections?
Frequently asked questions
- Order 25-OH-D, never 1,25-dihydroxy for routine screening.
- < 30 nmol/L is true deficiency and warrants loading.
- Above 50 nmol/L is sufficient for most adults.
- Do not test asymptomatic low-risk adults.
- D3 is preferred to D2.
References
3 sources- Endocrine Society2024 Clinical Practice Guideline on Vitamin D
Updated screening + supplementation guidance.
academic.oup.com
- USPSTFVitamin D Deficiency Screening
Grade I: insufficient evidence.
uspreventiveservicestaskforce.org
- NICE CKSVitamin D deficiency in adults
UK primary-care guidance.
cks.nice.org.uk
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