Vitamin D: what does 25(OH)D measure and what can a low result mean?

The blood test generally used to assess vitamin D status is 25-hydroxyvitamin D [25(OH)D]. The active hormone 1,25-dihydroxyvitamin D is regulated differently and is not the routine test for assessing vitamin D stores.

What can a low 25(OH)D mean?

Lower concentrations can reflect limited sun exposure, low dietary intake, malabsorption or other medical factors. Vitamin D is important for calcium and bone metabolism, but the meaning of a particular value depends on the clinical context.

Is there one “ideal” value?

Not necessarily. Different organisations have used different thresholds, and recent guidance emphasises that there is no universal serum concentration proven to be an optimal target for every healthy person. The NIH notes that concentrations of 20 ng/mL (50 nmol/L) or more are sufficient for most people for bone and overall health, while very low values are more clearly associated with deficiency.

Should everyone be tested?

Routine testing of all healthy adults is not universally recommended. Testing is more useful when there are clinical risk factors or a specific medical reason to assess vitamin D status.

Should I take a supplement?

The need, dose and duration of supplementation should not be decided from a single number alone. They depend on medical history, diet, age, coexisting conditions, bone-health risk and other factors. Excessive vitamin D intake can also cause adverse effects.

Scientific sources

1. NIH Office of Dietary Supplements — Vitamin D: Fact Sheet for Health Professionals.
2. Endocrine Society — Vitamin D for the Prevention of Disease: Clinical Practice Guideline.
3. Endocrine Society — Vitamin D Guideline: Patient Summary.

Last scientific review: 25 August 2026.