Biomarker · Nutrition

Vitamin D (25-hydroxyvitamin D)

The 25-hydroxyvitamin D test is the standard way to check vitamin D status. Learn what the result means, how units differ and what affects levels.

Written by LON6EVITY Editorial Team · Last updated

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Overview

Vitamin D supports calcium absorption and bone health. The blood test measures 25-hydroxyvitamin D, the main circulating form and the best indicator of vitamin D status according to the NIH Office of Dietary Supplements.

Why it matters

Deficiency can lead to weakened bones (osteomalacia in adults, rickets in children). Routine screening of everyone is not recommended by all guidelines; testing is often targeted at people with risk factors.

Higher and lower results

Low levels are common in winter at northern latitudes and in people with limited sun exposure, darker skin, obesity or malabsorption. Very high levels are usually due to excessive supplementation and can cause high calcium.

How the test is performed

A blood sample; no fasting required. Results are reported in nmol/L or ng/mL (1 ng/mL ≈ 2.5 nmol/L).

Reference ranges

NIH ODS: risk of deficiency< 30 nmol/L (< 12 ng/mL)
NIH ODS: adequate for most people≥ 50 nmol/L (≥ 20 ng/mL)
NIH ODS: potential adverse effects> 125 nmol/L (> 50 ng/mL)

Reference ranges and targets differ between laboratories, units and guidelines, and depend on age, sex, method and your overall clinical picture. A single result is not a diagnosis — interpret it with a qualified clinician.

Factors that can affect the result

  • Season and latitude
  • Sun exposure and skin type
  • Supplements
  • Body weight
  • Malabsorption

Questions people commonly ask

Is there one optimal vitamin D level?
Experts do not fully agree. The ranges above come from the NIH Office of Dietary Supplements; your clinician may use local guidance.

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Sources

  1. Vitamin D Test. MedlinePlus, U.S. National Library of Medicine.
  2. Vitamin D — Fact Sheet for Health Professionals. NIH Office of Dietary Supplements.