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Marker · Educational ranges, not diagnosis

25-hydroxy vitamin D

Also called 25-OH D, calcidiol, vitamin D blood test.

The liver storage form of vitamin D, and the test used to judge whether someone is short. It is not the active hormone 1,25-dihydroxy vitamin D.

Read this first

  • Coach, not doctor. Aaron Cuha is a coach, not a doctor. No clinical credential is claimed for him, or for these pages. A number on a panel is a conversation starter with a clinician, not a DIY diagnosis. Nothing here is a prescription, a protocol, or a reason to start or stop a medicine. Educational use only. This database summarizes published research and is not medical advice. Where a dose appears it is a record of what a trial or clinic ran, or what people describe doing, with its source attached. Nothing here is a recommendation for personal use.
  • Affiliate disclosure. Knowledge stays free. Every page, every guide and every tool is free to read and stays free. Lab-panel affiliate links, when they exist, live on /bloodwork/partners and print what is earned on the row, or that nothing is earned today. This glossary does not sell panels. The Archive does not order labs.
  • Three lab stories stay separate. /bloodwork is the human panel. /testing and /prices/labs are peptide certificate chemistry. /supplements/quality is what is in the bottle.

Why a longevity audience asks

Low levels track with dying sooner in huge cohorts. The largest randomised trials then gave vitamin D to people who were already replete and missed their primary endpoints. The useful move is to measure, not to assume every adult needs a bottle.

How a coach reads it

The owner's published conversation target is 40 to 60 ng/mL. Labs often flag under 20 ng/mL as deficient and 20 to 30 as insufficient. Those are conversation ranges, not a diagnosis you apply to yourself. Convert nmol/L to ng/mL by dividing by 2.5.

Units on this page: ng/mL, nmol/L

When it is low

Often means little sun, more indoor time, more body fat, or low intake. Repletion is a clinician conversation. The large prevention trials were not built to answer that question.

When it is high

Too high is a real problem, especially with high intermittent doses. Annual 500,000 IU increased falls and fractures in older women. More is not safer.

Easy ways to misread it

  • VITAL and D-Health enrolled people averaging about 31 ng/mL. Their nulls do not settle genuine deficiency.
  • If the dose is being pushed, calcium and PTH belong in the same conversation.
  • Vitamin D3 on a label is not the same as the blood test.

What to ask a clinician

Ask for 25-hydroxy vitamin D. If the number is very low or the dose is high, ask whether calcium and PTH should sit beside it.

Related supplements

Related when-playbooks

Sources on this page