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Answer · biomarkers

What does the vitamin D blood test measure?

Short answer

The serum 25-hydroxyvitamin D test measures the storage form of vitamin D and is the standard marker of vitamin D status. Sufficiency is generally cited at ≥30 ng/mL; many optimization frameworks target 40–60 ng/mL, individualized to the patient.

Reviewed by Dr. Laura PurdyMay 29, 2026

Full answer

The vitamin D blood test measures serum 25-hydroxyvitamin D (25-OH D), the circulating storage form, and is the standard marker of vitamin D status. The Endocrine Society defines deficiency below 20 ng/mL and insufficiency at 20–29 ng/mL; many longevity and metabolic frameworks target 40–60 ng/mL based on observational associations with immune, musculoskeletal, and metabolic outcomes. Vitamin D status is shaped by latitude, skin pigmentation, season, body composition, and supplementation. Repletion strategy is individualized: dose, form (D3 vs D2), and co-factors (magnesium, K2 in some frameworks) are provider-reviewed alongside calcium and PTH where indicated. Vitamin D is not a substitute for the broader biomarker panel — it is one input within it.

Educational. Not a substitute for individualized provider review.

References & sources
  • Vitamin D — Primary source · oMeds
  • Reviewed by Dr. Laura Purdy, MD, MBA · Clinical Director, oMeds · Last reviewed 2026-05-29

Related answers

Does vitamin D supplementation help?
Yes, when truly deficient. Repletion in confirmed vitamin D deficiency reliably raises serum 25-OH D into the sufficient range and improves musculoskeletal, immune, and fatigue markers in deficient patients. Supplementation in already-sufficient patients shows smaller and less consistent benefits.
Can low vitamin D cause fatigue?
Yes. Vitamin D deficiency is associated with fatigue, low mood, and reduced exercise tolerance, and randomized trials have shown improvement in fatigue scores with repletion. Vitamin D is rarely the only driver — it is read alongside iron, thyroid, and hormonal status.
What causes low vitamin D?
Vitamin D deficiency is driven by inadequate sun exposure, higher latitude and winter season, darker skin pigmentation, higher body fat (which sequesters vitamin D), older age, malabsorption, and insufficient dietary or supplemental intake.
Vitamin D vs K2?
They do different jobs. Vitamin D drives calcium absorption from the gut. Vitamin K2 directs that calcium into bone and away from arterial walls. The clinical case for routine co-supplementation is plausible but not yet definitively settled by randomized trials.
Primary source
Vitamin D