25-Hydroxyvitamin 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)

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.

Strong evidence · Last reviewed: 2025-06-30

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What 25(OH)D 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 & low levels

Causes of high 25(OH)D

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

Causes of low 25(OH)D

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.

Biomarkers

Homocysteine — B-vitamin and vitamin D status are often assessed together in fatigue and cognitive work-ups. Emerging evidence

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 — D3 supplementation reliably raises serum 25(OH)D; expected response 8-12 weeks. Strong evidence

Symptoms

Fatigue — Low vitamin D commonly presents with fatigue and muscle weakness. Moderate evidence

Frequently asked questions about 25(OH)D

What vitamin D level should I aim for?

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.

How much vitamin D should I take?

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.

Does VDR genotype mean I need a different dose?

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 & further reading

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.

Interpret your 25(OH)D in genetic context

This page is informational and not medical advice, diagnosis or treatment. Discuss lab results with a qualified healthcare professional before changing diet, supplements or medication.