Signs of low vitamin D: symptoms and testing explained

An empty blood collection tube on a yellow surface in low sunlight, standing for the vitamin D blood test

The short answer

Most people with low vitamin D notice nothing at all. The signs that are well documented, such as bone pain, muscle weakness, rickets in children and osteomalacia in adults, belong to severe and long standing deficiency¹². Tiredness, low mood, hair loss and frequent colds appear on every online list, but trials that corrected vitamin D did not reliably change them³. No symptom list can tell you your vitamin D level: only a 25(OH)D blood test can.

Does low vitamin D cause symptoms you can notice?

Often, no. Mild to moderate vitamin D deficiency is often silent, and many people with a low level feel no clear symptom they could trace to it. Low vitamin D is not rare, so this is not a small group. The European prevalence data support this: 13.0% of 55844 Europeans had a serum 25(OH)D below 30 nmol/L on average across the year⁴.

This is uncomfortable but important. A list of vague symptoms cannot separate a deficient person from a sufficient one, because tiredness and low mood are extremely common in people with normal vitamin D levels too. The reverse also does not hold: feeling fine does not prove your level is fine.

What is well documented is the spectrum at the far end. When deficiency is severe and lasts long, the body cannot maintain normal bone mineralisation, and that failure does produce recognisable signs¹². Those signs are covered in the next section.

Which signs belong to severe vitamin D deficiency?

Severe, long standing deficiency shows up as bone and muscle disease. In adults this is osteomalacia, with musculoskeletal pain, skeletal deformity, muscle weakness and, in some cases, symptomatic low calcium². In children the same mineralisation failure appears as rickets: thickened wrists and ankles, growth failure, bone pain, muscle weakness, waddling gait and leg bowing¹.

Osteomalacia from vitamin D deficiency is most often seen in housebound older people, immigrants to northern countries and people whose clothing covers most of the skin, as well as in malabsorption, liver disease and anticonvulsant therapy².

Older adults are a specific risk group because both production and metabolism of vitamin D change with age. A 2023 consensus statement on older adults sets two levels: avoid a serum 25(OH)D below 30 nmol/L, and aim for a level above 50 nmol/L⁵.

If you have bone pain, unexplained muscle weakness or signs of deformity, do not self diagnose with a supplement. See a doctor, because these signs have several possible causes and deserve a proper workup.

Why are tiredness, low mood and frequent colds unreliable signs?

Because they are common at every vitamin D level and have many causes. The clearest measurement comes from VITAL-DEP, which randomised adults aged 50 or older to 2000 IU vitamin D3 per day or placebo. The hazard ratio for depression or clinically relevant depressive symptoms was 0.97 with a 95% confidence interval of 0.87 to 1.09, and the mean difference in change in mood scores was 0.01 points³.

In other words: low vitamin D and low mood occur together in population data, but in this trial mood scores moved the same way in both groups. Association is not the same as causation, and this distinction is exactly why a symptom list is a poor diagnostic tool.

Fatigue, hair loss and frequent colds follow the same pattern. They are common, have many causes, and no large high quality trial has shown that vitamin D supplementation reliably removes them in the general population. That is not proof that no individual ever benefits, but it does mean these symptoms cannot be read as signs of low vitamin D.

Who is most likely to have low vitamin D in Europe?

Season, skin tone and age drive the numbers. In standardized European data, 13.0% of 55844 people had serum 25(OH)D below 30 nmol/L across the year, but this rose to 17.7% in extended winter (October to March) and fell to 8.3% in summer⁴. Under the alternative definition of below 50 nmol/L, prevalence was 40.4%⁴.

Group or season	Prevalence of low 25(OH)D	Definition used
Europe, whole year average	13.0%⁴	below 30 nmol/L
Europe, extended winter (October to March)	17.7%⁴	below 30 nmol/L
Europe, summer (April to November)	8.3%⁴	below 30 nmol/L
Europe, whole year average	40.4%⁴	below 50 nmol/L
Dark-skinned ethnic subgroups vs white populations	between 3 and 71 times higher prevalence⁴	below 30 nmol/L

Dark-skinned ethnic subgroups had a prevalence between 3 and 71 times higher than white populations, depending on the country and group studied⁴. Older adults are also at risk because skin production capacity and sun exposure both decline with age⁵. We covered the seasonal side of this in our summer versus winter vitamin D article.

Close-up of weak winter sunlight on a wool sleeve, the season when vitamin D levels in Europe drop

What does a vitamin D blood test actually measure?

A vitamin D test measures serum 25-hydroxyvitamin D, written as 25(OH)D, in nmol/L. It is the standard marker of vitamin D status because it reflects vitamin D from both sunlight and food. It is the marker the guidelines in this article use for their thresholds⁶⁷. It does not measure the active hormone calcitriol, and it does not measure how much vitamin D is stored in tissue.

The test is a single blood draw, usually requested by a doctor when there is a reason to check. If you want a broader view of what is worth checking before supplementing, including calcium and related markers, read our article on blood markers before supplementing.

What level counts as low?

Authorities disagree on where deficiency starts. UK guidance calls below 30 nmol/L deficient, while Polish guidance uses below 50 nmol/L and calls 75 to 125 nmol/L optimal⁶⁷. Here is the comparison.

Authority	Deficient	Insufficient or suboptimal	Sufficient or optimal
NOS guideline, UK⁶	below 30 nmol/L	30 to 50 nmol/L, may be inadequate in some people	above 50 nmol/L, sufficient for almost the whole population
Polish consensus 2023⁷	below 50 nmol/L	50 to 75 nmol/L	75 to 125 nmol/L
Consensus statement, older adults⁵	below 30 nmol/L to avoid	30 to 50 nmol/L	above 50 nmol/L as goal

The pattern is consistent: nearly every authority treats below 30 nmol/L as clear deficiency and above 50 nmol/L as adequate for bone health. The debate sits in the middle band and in how high to aim above 50 nmol/L.

When is a vitamin D test worth it, and when is it not?

A test is worth it when the result would change what you do: in risk groups, with symptoms of severe deficiency, before high dose use, or to check that a supplement works. Routine screening of healthy adults is explicitly not recommended, because no clinical trial evidence supports it⁸.

Situation	Is a 25(OH)D test worth it?	Why
Signs of severe deficiency such as bone pain or muscle weakness	Yes, via a doctor	These signs deserve full investigation, not just a vitamin D check²
Older age, dark skin, little sun exposure, malabsorption	Reasonable to discuss	These groups have much higher deficiency prevalence⁴⁵
Before or during high dose supplementation	Yes	Dose choice depends on your starting level⁶
Checking whether a supplement worked	Sometimes	Routine monitoring is generally unnecessary, but it can make sense with symptoms, malabsorption or doubt about whether you take it⁶
Healthy adult without risk factors, as routine screening	No	Guideline suggests against routine 25(OH)D testing in all populations considered⁸
Just tiredness or low mood, no other reason	Usually not	These symptoms have many causes, and a test result will not explain them on its own³

The 2024 Endocrine Society guideline states it plainly: no clinical trial evidence was found to support routine screening for 25(OH)D in the general population, and the panel suggests against routine 25(OH)D testing in all populations considered⁸. That is not a ban on testing individuals with a reason. It is a rejection of testing everyone.

If you test, when should you test again?

Not routinely. UK guidance states that routine monitoring of serum 25(OH)D is generally unnecessary, though it may be appropriate in symptomatic deficiency, malabsorption or suspected poor compliance⁶. One practical rule from the same guideline: serum calcium, adjusted for albumin, should be checked 1 month after completing a loading regimen or after starting supplementation, in case primary hyperparathyroidism has been unmasked⁶.

If you do retest to check a supplement, timing matters. Vitamin D status changes slowly, so give a new routine months, not weeks, before you read anything into a second result. For what to expect from timing in general, see how long before a supplement actually does anything.

What can a vitamin D supplement claim, and what can it not?

A vitamin D supplement can carry authorised claims, and these are the ones our drops carry. Vitamin D contributes to the normal function of the immune system. Vitamin D contributes to normal absorption/utilisation of calcium and phosphorus. Vitamin D contributes to the maintenance of normal bones. Vitamin K contributes to the maintenance of normal bones. Vitamin K contributes to normal blood clotting. The EU register holds a few more for vitamin D, and every one of them is about normal body function, none about treating or preventing disease⁹.

Tiredness, mood, hair and colds are not on that list. Wording that goes beyond the authorised list is not an authorised claim, whoever prints it.

If you do supplement, dose discipline matters. Our Vitamin D3 + K2 drops provide 125 microgram (5000 IU) vitamin D3 plus 120 microgram vitamin K2 as MK-7 per 0.25 ml serving, taken once every 3 days, which averages about 42 microgram a day by our calculation. EFSA sets the tolerable upper intake level for adults at 100 microgram a day¹⁰. The drops are not intended for pregnant or breastfeeding women, and if you take anticoagulants (blood thinners), consult your physician before use. Vitamin D works slowly on status, so consistency beats bursts.

A person in a winter coat seen from behind on a city street in low November light

Frequently Asked Questions

Can you feel low vitamin D?

Usually not. Mild to moderate deficiency is often silent, and the clear signs, such as bone pain and muscle weakness, come with severe, long standing deficiency¹².

Is tiredness a sign of low vitamin D?

Tiredness alone is not a reliable sign. It is extremely common regardless of vitamin D status, and trials correcting vitamin D did not reliably relieve it in the general population³.

Which vitamin D test should you ask for?

Ask for serum 25-hydroxyvitamin D, written as 25(OH)D. It is the marker the guidelines in this article use for their thresholds, and the result is given in nmol/L or ng/mL⁶⁷.

Should healthy adults get their vitamin D tested?

No. The 2024 Endocrine Society guideline found no clinical trial evidence for routine screening and suggests against routine 25(OH)D testing in all populations considered⁸.

What is a normal vitamin D level?

Most authorities call above 50 nmol/L sufficient for almost everyone⁵⁶. Polish guidance goes further and calls 75 to 125 nmol/L optimal⁷.

The Bottom Line

Low vitamin D is common in Europe, with 13.0% of people below 30 nmol/L on average and 40.4% below 50 nmol/L⁴, but it is mostly silent. The documented signs belong to severe deficiency and warrant a doctor, not a self test. Tiredness and low mood are too common to point to vitamin D, and in a large trial mood scores were the same with vitamin D as with placebo³. Test when the result would change your action: risk groups, severe symptoms, high dose plans or checking a supplement. Skip routine screening, which the 2024 Endocrine Society guideline advises against⁸. If you supplement, the authorised claims for vitamin D are about normal bones, calcium and immune function.

What are the signs of low vitamin D, and when is a test worth it?