Vitamin D (25-Hydroxyvitamin D)
Vitamin D (measured as 25-hydroxyvitamin D or 25(OH)D) is a fat-soluble vitamin that functions more like a hormone in the body. It plays critical roles in calcium absorption, bone mineralization, immune regulation, muscle function, and mood. Vitamin D deficiency is one of the most common nutritional deficiencies worldwide.
The body produces vitamin D when skin is exposed to UVB sunlight, but factors like latitude, season, skin pigmentation, sunscreen use, and time spent indoors mean many people don't produce enough. Food sources (fatty fish, fortified dairy) provide relatively small amounts compared to sun exposure or supplementation.
Research increasingly links adequate vitamin D levels to reduced risk of autoimmune conditions, respiratory infections, cardiovascular disease, and certain cancers. The optimal range for health optimization is debated, but most functional medicine practitioners recommend maintaining levels above 40 ng/mL.
What It Measures
The 25-hydroxyvitamin D test measures the main circulating form of vitamin D in the blood. It reflects combined vitamin D intake from sun exposure, diet, and supplements over the past 2–3 weeks.
Reference Ranges
| Status | Range (ng/mL) |
|---|---|
| Optimal | 40–60 ng/mL |
| Normal | 30–40 ng/mL |
| Borderline | 20–30 ng/mL (insufficient) |
| Out of Range | < 20 ng/mL (deficient) |
Optimal Ranges
| Group | Range | Unit | Source |
|---|---|---|---|
| Adults | 30–100 | ng/mL | Standard reference |
| Optimal (functional) | 50–80 | ng/mL | Functional range |
When Values Are High
Vitamin D toxicity (>100 ng/mL) can cause hypercalcemia, nausea, kidney stones, and vascular calcification. Rare from sun exposure — almost always from excessive supplementation.
When Values Are Low
Associated with bone loss, immune dysfunction, fatigue, depression, muscle weakness, and increased infection risk. Deficiency is widespread, especially at northern latitudes and in darker-skinned individuals.
What To Do
Lifestyle
- •20–30 minutes of midday sun exposure 3–4x/week (latitude-dependent)
- •Include fatty fish, egg yolks, and fortified foods
- •Test in late winter when levels are lowest
Supplements
- •Vitamin D3 (cholecalciferol) 2,000–5,000 IU daily if deficient
- •Take with fat-containing meal for absorption
- •Co-supplement with K2 (MK-7) to direct calcium to bones, not arteries
When to Retest
Retest 8–12 weeks after dose adjustment
How GATOR Tracks This
Lab Providers
Related Biomarkers
This content is for informational purposes only and does not constitute medical advice. Reference ranges may vary by laboratory and individual factors. Consult a qualified healthcare provider for interpretation of your results and treatment decisions.
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