What should vitamin D be?

What does the 25 OH vitamin D test measure?

25-hydroxy vitamin D, 25(OH)D, is the main indicator of vitamin D status. It reflects the vitamin produced in the skin together with the amount obtained from foods and supplements. The NIH reports that it has a half-life of about 15 days in serum. This form, measured in routine assessment, and the active form, 1,25(OH)₂D, are not used interchangeably. ^[1]

Names such as “25-OH vitamin D,” “25(OH)D total,” or “25-hydroxy vitamin D” may appear on the report. Total 25(OH)D is the sum of 25(OH)D₃ and 25(OH)D₂; some studies, such as the Tokat study, measured only the D₃ component. The international consensus of Giustina and colleagues defines total serum 25(OH)D as the accepted biomarker of vitamin D status. ^[5]

What should the vitamin D level be? Thresholds by organization

Thresholds vary according to which outcome is being prevented and which group they are applied to. The NIH explicitly states that the levels at which 25(OH)D indicates deficiency, adequacy, and general health have not been definitively established; the Giustina consensus also explains that threshold differences stem from different clinical and public health perspectives. For example, for osteomalacia, the IOM chose the threshold at which 97.5% of participants had no osteomalacia (public health perspective), while the Endocrine Society chose the threshold at which no one had it (clinical perspective). ^{[1] [5]}

Three conclusions follow from the table. First, 20 ng/mL is a shared boundary in most approaches: NIH/NASEM considers below it inadequate, and the Türkiye consensus considers it deficient. Second, the roughly 20–29 ng/mL range is called “adequate” (NASEM; in Türkiye, for bone-muscle) or “insufficient” (Endocrine Society 2011) depending on the organization; the same result can receive different labels. Third, higher does not mean more beneficial: the NIH considers 20 ng/mL and above adequate for most people and associates levels above 50 ng/mL with possible adverse effects. The Türkiye consensus cites 30–50 ng/mL as the range proposed to be sufficient for non-skeletal benefits such as immune regulation and cardiometabolic functions, and states that 50–60 ng/mL provides no additional benefit in most people; as seen on the benefits page, randomized trial results demonstrating these non-skeletal benefits in the general population are limited. For this reason, the 30–50 ng/mL range should be read as an expert recommendation, not as a proven target. ^{[1] [4]}

How do the Endocrine Society’s 2011 and 2024 positions differ?

The 2011 guideline recommended 25(OH)D measurement as the initial diagnostic test in people at risk and supported treatment with D2 or D3 in those with deficiency; it did not recommend screening in people without risk or prescribing vitamin D for cardiovascular protection, on the grounds that there was insufficient evidence. ^[2]

The 2024 guideline answers an entirely different question: in people without a clear indication for vitamin D treatment or a 25(OH)D test, what should be done to reduce disease risk? The panel recommended against empiric supplementation above the reference intake in healthy adults under 75 and against routine 25(OH)D testing in all groups evaluated; it stated that there is no clear evidence defining an optimal target level for disease prevention. It did recommend empiric supplementation at ages 1–18, over 75, in pregnancy, and in high-risk prediabetes. The guideline was not written to replace DRI values and does not cover those with diagnosed deficiency or an indication for testing. ^[3]

That is, the 2024 guideline does not say it “lowered the threshold” or “20 ng/mL is adequate”; it only does not find it proven to decide on supplementation or testing in healthy people by looking at the number.

How are ng/mL and nmol/L converted?

The NIH reports that 1 ng/mL equals 2.5 nmol/L and 1 nmol/L equals 0.4 ng/mL; the exact factor based on molecular weight is 2.496. In Türkiye, reports are mostly given in ng/mL; in European sources and some laboratories, nmol/L is used. The Giustina consensus states that the molar unit is the SI standard and recommends reporting both units together where possible. ^{[1] [5]}

The purpose of conversion is to compare the number on paper with the correct threshold. 25 nmol/L and 25 ng/mL are not the same value; the latter is about 62 nmol/L. If a result is interpreted without checking the unit, deficiency and adequacy can be confused. ^[1]

How reliable is a laboratory result?

25(OH)D measurement can vary from laboratory to laboratory. Giustina and colleagues state that measurement can be done with antibody-based methods (chemiluminescence, immunoenzymatic) or with liquid chromatography–mass spectrometry (LC-MS/MS), that LC-MS/MS gives more consistent and accurate results, but that method standardization is still an important problem. In the CDC’s 2019 report, the mean individual-sample pass rate (±5% deviation criterion) over 40 serum samples of 20 immunoassay and 17 LC-MS/MS methods certified as “standardized” was 30% for immunoassays and 61% for LC-MS/MS methods; that is, even with standardized methods not every sample meets the target. For this reason, results from different laboratories or methods should not be compared one to one; the report should state the method, unit, and reference material. ^[5]

The Türkiye consensus also considers LC-MS/MS the preferred method and, when it is not available, accepts validated immunoassays compatible with international standards such as VDSP (Vitamin D Standardization Program). It is also reported that 25(OH)D levels in Türkiye vary by season, with peak values seen in the late spring and summer months in observational data; for this reason, the season in which the test was done is also part of interpretation. ^[4]

A laboratory’s “reference range,” on the other hand, is often a population-based statistical range; it should not be read alone as a treatment target or threshold. The result is interpreted by a physician, stating which threshold source (NIH/NASEM or the Türkiye consensus) is used.

How are thresholds used in Türkiye data?

The same Türkiye data are read differently depending on the threshold used. Based on meta-analyses and national hospital data, the consensus reports that roughly 60% of adults have a level below 20 ng/mL; prevalence and risk groups are covered on the deficiency symptoms page. ^[4]

The population-based study of 2,225 adults in Tokat classified the 25(OH)D₃ level as below 10 ng/mL (deficient), 10–20 ng/mL (insufficient), and above 20 ng/mL (sufficient): 38.7% were found deficient and 46.4% insufficient. The sum of the two categories (85.1%) corresponds to the proportion below 20 ng/mL; however, in this study a sample different from hospital data and a different component (D3) were measured. For this reason, the numbers are not used interchangeably; the result from a single province cannot be carried over to all of Türkiye. ^[6]

What does a high value mean?

The NIH reports that levels above 50 ng/mL (125 nmol/L) may be associated with possible adverse effects, with this risk emphasized especially above 60 ng/mL (150 nmol/L). The Türkiye consensus classifies 100 ng/mL and above as hypervitaminosis D and 150 ng/mL and above as intoxication associated with hypercalcemia and kidney-cardiovascular complications; it also states that with chronic high-dose supplementation, hypercalciuria or kidney stones can develop at lower levels. The same document does not consider 50–60 ng/mL consistent with toxicity but regards it as above target and recommends reviewing total intake; for 60–99 ng/mL, it mentions a 2–3 month break from supplementation with re-measurement after about 8–12 weeks. These are clinical steps assessed by a physician; they are not a decision to stop the product or change the amount on one’s own. ^{[1] [4]}

With a high result, all products used (multivitamin, calcium-D combinations, fish oil products) and the assessment related to calcium should be considered together.

When is the test repeated?

The Türkiye consensus recommends re-measuring 25(OH)D 8–12 weeks after starting treatment or a loading regimen, because steady state is reached in this interval. Depending on the result, the regimen is continued, adjusted, or moved to maintenance; subsequent follow-up is determined by the baseline situation and ongoing risk factors. ^[4]

Testing a few days after a new supplement has limited informational value; the roughly 15-day half-life reported by the NIH means that the level responds to a change within weeks. Routine repeat testing is not recommended in a person without symptoms or risk who has not started a supplement. ^{[1] [3]}

Does correcting a low level always provide more benefit?

No. In the individual participant data meta-analysis by Martineau and colleagues, the protective effect against respiratory tract infections was marked in those with a baseline level below 25 nmol/L (10 ng/mL) (OR 0.30); it was smaller in those at 25 nmol/L and above (OR 0.75). This is a finding suggesting that correction may be beneficial at very low levels; however, it does not show that raising the level higher in a healthy person with an adequate level provides benefit, and it does not set a target level. ^[7]

In which outcomes supplementation is proven is a separate topic and is covered on the benefits page. This page does not recommend a personal dose or target level.

2025 update: respiratory infections

The small protective signal reported in 2021 was no longer statistically significant in the 2025 update. The main placebo comparison included 40 trials and 61,589 participants: OR 0.94 (95% CI 0.88–1.00; p=0.057). There was no reliable evidence that effects differed by age, baseline vitamin D status or dosing regimen. These findings do not support extra vitamin D for everyone to prevent infection; they do not remove the need to treat diagnosed deficiency. The earlier 2017 and 2021 findings should be read alongside this update. ^[8]

Guideline clarification: daily requirements versus extra supplementation

The Endocrine Society’s correction explicitly states that the 2024 guideline replaces its 2011 guideline. For generally healthy adults under 50 and aged 50–74, the recommendation against empiric or routine supplementation concerns intake beyond the recommended dietary reference intake. It does not mean neglecting daily requirements or leaving diagnosed deficiency untreated. The 2011 thresholds should be read as historical guidance, not current universal targets. ^[9]