Low vitamin D: what the tests show and why the values must be read carefully
Written by
Redazione Qura
Medically reviewed by
Dott.ssa Gabba · Internal medicine
Category
Wellbeing
The Qura editorial team curates and fact-checks health content together with the doctors in the Qura network, with the goal of making test results understandable and useful.
In blood tests vitamin D is measured as 25-OH vitamin D (calcidiol), the form that reflects the body’s stores. A low value indicates reduced reserves — most often from little sun exposure or reduced intake — not a disease in itself, and should be interpreted together with calcium, phosphorus and parathyroid hormone and over time.
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In blood tests vitamin D is measured as 25-OH vitamin D (calcidiol): it is the form that reflects the body’s stores. A low 25-OH vitamin D value indicates reduced reserves, most often due to little sun exposure or reduced intake, not a disease in itself. It often needs to be interpreted together with calcium, phosphorus and parathyroid hormone.
What 25-OH vitamin D measures (and why 1,25-OH is not tested)
When vitamin D is requested, the laboratory measures 25-hydroxyvitamin D, indicated as 25(OH)D or 25-OH vitamin D. It is the marker recognized internationally as a reliable indicator of vitamin status, because it represents the sum of what is produced by the skin with sunlight and what is taken in through food and supplements.
The active form of the hormone, 1,25-OH vitamin D (calcitriol), is normally not measured in screening: its levels often remain normal even in deficiency, because the body maintains them at the expense of the reserves. This is why measuring 1,25-OH should be reserved for specialist situations, on the doctor’s indication, and is not useful for establishing whether the stores are low.
Low vitamin D: which values are considered a deficiency
The Italian SIOMMMS 2023 guidelines indicate as desirable in adults 25(OH)D values between 20 and 40 ng/mL, and consider a value below 20 ng/mL indicative of deficiency. The threshold is not universal: for those with osteoporosis or other bone diseases the desirable level rises to 30 ng/mL. The table summarizes the most-used references.
| 25-OH vitamin D | Equivalent (nmol/L) | How it is usually read |
|---|---|---|
| below 20 ng/mL | below 50 nmol/L | Deficiency (vitamin D deficiency) according to the SIOMMMS 2023 guidelines |
| 20-40 ng/mL | 50-100 nmol/L | Desirable range in a healthy adult |
| below 12 ng/mL | below 30 nmol/L | Threshold below which AIFA Note 96 provides for reimbursement of supplementation in the absence of other risk conditions |
| 30 ng/mL or more | 75 nmol/L or more | Desirable level for those with osteoporosis or other established bone diseases |
The numbers should be taken with caution: the ranges depend on the laboratory method, which for vitamin D is notoriously poorly standardized. The only valid reference for your result is the one printed next to the value on your report.
Why 25-OH values must be read carefully
The reference range is not a disease threshold, but a convention: it describes what is common in a population, not what is healthy for the individual person. With vitamin D this distinction matters more than elsewhere, because the thresholds are debated among scientific societies.
A concrete example: the same value can be classified in different ways. The SIOMMMS guidelines place deficiency below 20 ng/mL, while AIFA Note 96, which regulates the reimbursement of medicines, has lowered to 12 ng/mL the threshold below which supplementation is started in those without other risk conditions. These are not contradictory numbers: they answer different questions (when the stores are low, when it makes sense to treat at the expense of the health service).
Translated: a 25-OH slightly below the limit, in a person without symptoms and without risk factors, usually does not equate to a clinical problem. The useful reading starts from the context, not from the asterisk on the report.
What lowers vitamin D
The most common cause of a low value is poor skin synthesis: the skin produces vitamin D only with exposure to UVB rays, which at our latitudes are scarce in autumn and winter. This is why values are physiologically lower in the cold months and in those who spend little time outdoors, use a lot of sun protection or have darker skin.
Advanced age also weighs in (the skin synthesizes less), as do reduced dietary intake, some conditions of intestinal malabsorption and the increased need in pregnancy and breastfeeding. Obesity too is associated with lower values, because vitamin D is sequestered in adipose tissue. These are associations, not automatisms: the weight of each factor must be assessed by the doctor case by case.
Which other tests vitamin D should be read with
A 25-OH value taken on its own says little. To understand whether a deficiency has consequences on bone metabolism, it is read together with calcium, phosphorus and above all parathyroid hormone (PTH): when vitamin D stores drop, PTH tends to rise to maintain calcium in the blood, and it is precisely this combined finding that makes the data clinically significant.
Depending on the picture, the doctor may add alkaline phosphatase, kidney function (creatinine and eGFR) and, in selected cases, urinary calcium. It is the cross-check between these parameters, not the single vitamin D number, that guides the interpretation. This is where the value of an integrated reading lies: related markers looked at together, and compared with previous values. If you want to understand how a report is read as a whole, start from the guide on how to read blood test results.
When a low value is clinically relevant and should be investigated with the doctor
It is worth taking the report to the doctor when 25-OH is markedly low (and not just slightly below the limit), when it is accompanied by altered calcium or phosphorus or by an elevated PTH, when there are conditions that justify further investigation (osteoporosis, malabsorption, chronic therapies) or when the value worsens compared with previous checks.
The most informative data is often the trend: a stable 25-OH, even if not optimal, tells a different story from one that falls test after test. The decision on how to intervene, and whether to intervene, remains the doctor’s, who knows your history and the full picture.
Sources
Frequently asked questions
What is the difference between vitamin D and vitamin D3?
Vitamin D3 (cholecalciferol) is the form produced by the skin with sunlight and contained in some foods; it is the starting point. In tests, however, D3 is not measured as such, but rather 25-OH vitamin D, which is its transformation in the liver and represents the reserves. A report with low vitamin D3 and one with low 25-OH therefore indicate the same thing: reduced stores.
Does low vitamin D cause symptoms?
Often a mild or moderate deficiency is silent and is only discovered through tests. Any complaints are non-specific and overlap with many other causes, so they are not enough to make a diagnosis. If you suspect a low value is linked to how you feel, it is a discussion to bring to your doctor, not a conclusion to draw from the report.
Should a 25-OH value just below the reference worry me?
Usually not. A value slightly below the limit, in a person without risk factors and without other altered tests, often falls within normal variability, especially in the winter months. What counts is the overall picture and the trend over time, not the single isolated number.
Why is my value different from the references I find online?
Because vitamin D measurement is not yet fully standardized: different laboratories and methods can return slightly different ranges. The only valid reference for interpreting your result is the one shown next to the value on your report.
Tags
- low vitamin d
- 25-OH vitamin D
- vitamin d deficiency
