Vitamin D
25-hydroxyvitamin D, 25(OH)D
25-hydroxyvitamin D is the storage form of vitamin D and the value labs report. It reflects your supply over the past weeks rather than today.
Reference range
ng/mL- Women
- 30–70 ng/mL
- Men
- 30–70 ng/mL
Reference ranges differ between laboratories and assays. Always read your result against the range printed on your own report.
What Vitamin D measures
Vitamin D exists in several forms, and the one measured is 25-hydroxyvitamin D. It is the storage form: made in the liver from what your skin produced under sunlight or what you took as a supplement, and circulating with a half-life of two to three weeks.
That half-life is the reason this form is measured rather than the active one. Active vitamin D, calcitriol, is tightly regulated and can look perfectly normal while stores are empty. 25(OH)D shows the actual supply.
Levels follow a seasonal curve at higher latitudes. Above roughly the 40th parallel, skin synthesis effectively stops from autumn to spring, so a test in February and a test in August can differ substantially in the same person.
Why it gets tested
To investigate bone pain, muscle weakness, frequent infections or fatigue, and as part of assessing osteoporosis risk. Also to check whether supplementation is achieving what it was meant to.
Commonly associated with low values
- Little sun exposure, from latitude, season, indoor work or covering clothing
- Darker skin, which needs considerably longer exposure for the same synthesis
- Malabsorption, since vitamin D is fat-soluble
- Higher body weight, which distributes the same amount across more tissue
Commonly associated with high values
- Supplementation above requirements — the usual cause by a wide margin
- Rarely, granulomatous conditions such as sarcoidosis
What can shift the result
- The season of the blood draw can shift a result more than most interventions do
- Assays differ between laboratories, so values are not always directly comparable
- Where the threshold for deficiency lies is genuinely contested between medical bodies
Questions
Why do vitamin D reference ranges vary so much?
Because expert bodies disagree about what the target should be. Some define sufficiency from 20 ng/mL, others from 30 ng/mL, and different assays produce slightly different numbers for the same sample. A result near a threshold should be read as approximate rather than as a verdict.
Does the time of year affect the result?
Considerably, at higher latitudes. Skin synthesis largely stops between autumn and spring, so levels typically peak in late summer and reach their lowest point in late winter. Comparing a February value with the previous August's is comparing two different seasons, not two different states of health.
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This page is general information about a laboratory value. It is not a diagnosis and does not replace medical advice. Discuss your own results with a doctor, who can read them in the context of your history and symptoms.