25-Hydroxyvitamin D (25-OH D)

The vitamin D test to ask for is 25-hydroxyvitamin D, written 25-OH D or 25(OH)D on the report. It measures calcidiol, the storage form your body banks and later draws on, so it reflects your actual reserves. Below 20 ng/mL (50 nmol/L) is the level most guidelines call deficient.

What should your vitamin D level be?

Most US labs and guidelines put the deficiency line at 20 ng/mL (50 nmol/L) and treat 30 to 50 ng/mL (75 to 125 nmol/L) as a comfortable target, with under 12 ng/mL (30 nmol/L) marking clear deficiency. ¹² Find the line labelled 25-hydroxyvitamin D, 25-OH D, or 25(OH)D on your report, note the unit, and if the number sits under 20 ng/mL or over 100 ng/mL (250 nmol/L), bring it to a clinician and ask whether a repeat test in about 3 months makes sense.

These bands follow the 2011 Endocrine Society guideline. ² The Institute of Medicine sets the bar lower, calling 20 ng/mL (50 nmol/L) enough for about 97.5% of healthy people and reserving deficient for under 12 ng/mL (30 nmol/L). ¹

Why 20, 30, or no number at all

The two numbers come from different questions. The Institute of Medicine (2011) modelled bone health across the population and concluded 20 ng/mL (50 nmol/L) covers nearly everyone. ¹ The Endocrine Society's 2011 guideline set a higher bar, 30 ng/mL (75 nmol/L), aimed at patients already at risk rather than the general public. ² In 2024 the Endocrine Society went further and suggested against routine 25-hydroxyvitamin D testing in healthy adults, declining to name a target level for the general population and recommending vitamin D without testing only for select groups such as adults over 75, pregnancy, children and teens, and high-risk prediabetes. ³ The US Preventive Services Task Force reached a similar place in 2021, finding the evidence insufficient to recommend screening adults who have no symptoms. ⁴

The current evidence is insufficient to assess the balance of benefits and harms of screening for vitamin D deficiency in asymptomatic adults. ⁴US Preventive Services Task Force, JAMA, 2021

Normal is not the same as optimal

A result inside the lab's printed reference range means it is common, not that it is ideal for you; ranges are built from how a reference population tested, then flagged at the statistical edges. See how reference ranges are built for why in range and optimal can differ. With vitamin D the disagreement is baked into the cutoff itself, so two labs can flag the same 25 ng/mL differently. For the wider picture, see all vitamin and mineral markers.

What is 25-hydroxyvitamin D?

Vitamin D from sun, food, or a supplement is not active yet. The liver converts it to 25-hydroxyvitamin D (25-OH D, also called calcidiol), the form that circulates and gets stored, with a half-life (the time for half of it to clear) of about 2 to 3 weeks. ⁶ That slow turnover is what makes it a good gauge of your reserves: it tracks your overall supply, not the last meal or the last hour of sun.

25-OH D vs the active form: which test reads your status

The body makes a second, active form in the kidney: 1,25-dihydroxyvitamin D (calcitriol). It is the hormone that raises calcium, but it is the wrong thing to measure for status.

The active number misleads because a hormone feedback loop keeps it topped up exactly when your stores are running low. ²

Its half-life is only about 4 to 6 hours, and it is steered by parathyroid hormone (PTH) and FGF23 (a hormone that manages phosphate), so it tracks minute-to-minute signalling, not how much vitamin D you have banked. ⁶ The active 1,25-dihydroxyvitamin D test has real uses, such as certain calcium and kidney conditions, but reading vitamin D status is not one of them. ²

Reading the units: ng/mL vs nmol/L

US labs report vitamin D in nanograms per milliliter (ng/mL); much of Europe uses nanomoles per liter (nmol/L). To convert, multiply ng/mL by 2.5. So 20 ng/mL is 50 nmol/L, 30 ng/mL is 75 nmol/L, and 50 ng/mL is 125 nmol/L. ¹⁵ A result of 40 is deficient on the European scale (40 nmol/L, about 16 ng/mL) but sufficient on the US one (40 ng/mL), so always check which unit your report uses before comparing it to a target.

Do you have to fast, and who should test?

No fasting is needed for a 25-hydroxyvitamin D test, and it is not part of a standard metabolic or cholesterol panel, so it is usually ordered on its own. It is inexpensive and widely available. Testing makes the most sense when there is a reason to expect a low level.

• Limited sun exposure, or consistently covered skin
• Darker skin, which makes less vitamin D per minute of sun
• Older age, especially over 65 to 70
• Higher body weight or obesity, which lowers circulating 25-OH D
• Conditions that block fat absorption, such as celiac disease, Crohn's disease, or after weight-loss surgery
• Osteoporosis, or a bone-thinning workup

These are the same at-risk groups that current guidelines point toward, rather than the general public. ²³ If a test does come back low and you start a change, the number moves slowly, which sets the timing for a recheck.

That slow climb is why a repeat test is usually timed for about 3 months after any change, not a few weeks in: check too early and the number has not settled yet.

How to raise a low vitamin D level

The usual routes are more sun, vitamin D from food (fatty fish like salmon, egg yolks, fortified milk and cereal), and supplements. ⁵ How much you need depends on your starting number, your weight, and why it is low, which is why the amount is a conversation with a clinician rather than a fixed dose off a label. If your 25-OH D is low, the parent page on vitamin D deficiency covers the common causes, symptoms, and how it is managed.

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