# 25-Hydroxyvitamin D
URL: https://genohealth.app/biomarkers/25-oh-vitamin-d
Units: ng/mL (US) · multiply by 2.5 for nmol/L
Functional optimal: 40-60 ng/mL (functional optimal cited by many integrative groups)
Clinical reference: Sufficient ≥ 30 ng/mL · Deficient < 20 ng/mL (Endocrine Society)
Evidence: Strong evidence
Last reviewed: 2025-06-30

## Summary
25-hydroxyvitamin D is the best single measurement of vitamin D status — and one of the most personalized labs, because VDR and CYP2R1 genetics meaningfully change how much you need.

## What it is
25-hydroxyvitamin D is the storage form of vitamin D and the marker that best reflects long-term vitamin D status, integrating both sun-derived and dietary/supplement sources.

## Why it matters
Vitamin D regulates calcium balance and bone health, supports immune function, and influences mood and muscle strength. Severe deficiency causes osteomalacia and rickets; insufficiency is widely associated with falls, fractures, and worse outcomes across many conditions.

## Causes of high
- High-dose supplementation, especially if VDR variants reduce response
- Granulomatous diseases (sarcoidosis, tuberculosis)
- Very rare genetic conditions affecting metabolism

## Causes of low
- Limited sun exposure, indoor lifestyle, latitude > 35°
- Darker skin (higher melanin reduces cutaneous synthesis)
- Older age — less efficient skin synthesis
- Obesity — vitamin D is sequestered in adipose tissue
- Malabsorption (celiac, IBD, after bariatric surgery)
- CYP2R1 variants — reduced 25-hydroxylation
- VDR variants alter tissue response without always changing 25(OH)D

## Lifestyle
- Short midday sun exposure (10-20 min on arms/legs, latitude-dependent) — without burning
- Resistance training supports muscle and bone alongside vitamin D

## Nutrition
- Fatty fish (salmon, sardines, mackerel) — best dietary sources
- Egg yolks, fortified dairy or plant milks
- Most adults need 1000-4000 IU/day D3 to maintain optimal levels; some need more — re-test 8-12 weeks after starting
- Pair D3 with vitamin K2 (MK-7) and adequate magnesium for proper utilization

## Testing notes
Ask specifically for 25-hydroxyvitamin D (25(OH)D), not 1,25-dihydroxy. Re-test every 8-12 weeks when adjusting dose.

## Related (knowledge graph)

### Biomarkers
- Homocysteine (/biomarkers/homocysteine): B-vitamin and vitamin D status are often assessed together in fatigue and cognitive work-ups. [Emerging evidence]

### Genes
- VDR (/genes/vdr): VDR variants (FokI, BsmI, TaqI) influence tissue response to vitamin D and may shift the optimal target range. [Moderate evidence]

### Supplements
- Vitamin D3 (/supplements/vitamin-d3): D3 supplementation reliably raises serum 25(OH)D; expected response 8-12 weeks. [Strong evidence]

### Symptoms
- Fatigue (/symptoms/fatigue): Low vitamin D commonly presents with fatigue and muscle weakness. [Moderate evidence]

## FAQ
**Q: What vitamin D level should I aim for?**
A: 30 ng/mL is the conventional minimum. Many functional practitioners target 40-60 ng/mL based on observational data and individual symptom response. Above 100 ng/mL is unnecessary and approaches the range where toxicity becomes possible.

**Q: How much vitamin D should I take?**
A: Most adults need 1000-4000 IU/day of D3 to maintain optimal levels — but the right dose depends on baseline, body weight, sun exposure, and genetics (VDR, CYP2R1). Re-test 8-12 weeks after starting to confirm response.

**Q: Does VDR genotype mean I need a different dose?**
A: Sometimes. VDR variants don't always change your blood 25(OH)D level dramatically, but they can change how strongly your tissues respond. Some clinicians target the higher end of optimal (50-60 ng/mL) for VDR FokI carriers.

## Citations
- Holick et al., J Clin Endocrinol Metab 2011 — Endocrine Society Guideline (PMID 21646368): Defines deficiency at < 20 ng/mL and sufficiency at ≥ 30 ng/mL; many specialists now target higher.
- Uitterlinden et al., Gene 2004 — VDR polymorphisms (PMID 15315756): VDR variants (FokI, BsmI, TaqI) influence tissue response to vitamin D and downstream outcomes.

Disclaimer: Informational only — not medical advice.