Explainer · updated 2026-09-09

Vitamin D, explained in plain English

Written by the medk.AI team · Last reviewed September 2026 by the medk.AI content team

Key facts

• Typical adult reference interval: 30-60 ng/mL (ABIM)
• The same interval in nmol/L: 75-150 nmol/L (ABIM)
• Level below which a result counts as deficiency: 20 ng/mL (HOLICK 2011)
• Levels of 21 to 29 ng/mL are classed as: insufficiency, on the thresholds most labs still use (HOLICK 2011)
• What the test measures: both storage forms of vitamin D added together, the one the body makes in skin and the plant-derived one found in some supplements (HOLICK 2011)

What it is

The test usually labelled "Vitamin D, total" counts both storage forms of vitamin D together: the one the body makes in skin and the plant-derived one found in some supplements. Adding them is what most laboratories now do, and the thresholds apply to the total. The result is measured in the blood.

What the number is telling you

The level reflects how much vitamin D the body has stored. Below 20 ng/mL counts as deficiency, and 21 to 29 ng/mL as insufficiency, on the thresholds most labs still use. Sunlight on skin is where most vitamin D comes from, and too little of it is the main reason people run short. Where your own level sits is for your doctor to say.

What high or low can mean

Low: Below 20 ng/mL counts as deficiency, and 21 to 29 ng/mL as insufficiency, on the thresholds most labs still use. Factor 30 sunscreen cuts what the skin makes by over 95 per cent, and darker skin needs three to five times longer in the sun, so how the level is reached matters as much as the number itself.

High: Above 150 ng/mL is vitamin D toxicity, which practically only happens through very large supplement doses. A high level with no supplement use behind it is worth raising with your doctor.

What a typical result looks like

Typical adult reference intervals from ABIM run from 30 to 60 ng/mL, about 75 to 150 nmol/L. The range on your own report is the one that counts.

Questions worth asking your doctor

• Is my result in ng/mL or nmol/L, and where does it sit in the interval?
• If my level is low, is a supplement right for me, and at what dose?
• How long should I wait before retesting?
• How does this compare with my previous results?

If a result worries you, ask your doctor.

Sources

• Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline (2011)