- 01The number moves before you feel anything
Vitamin D deficiency is defined by the blood level, a 25(OH)D under 20 ng/mL, and it can be present with no symptoms at all. Most people with a low level feel nothing, so waiting to 'feel deficient' means the marker can sit under the line for months unnoticed. Fatigue or aches are late and nonspecific, so a 25(OH)D blood test is what confirms it.
- 02'Normal' and 'optimal' don't agree
The Institute of Medicine calls 20 ng/mL adequate for almost everyone, while the 2011 Endocrine Society set 30 ng/mL as sufficient. The same result can be flagged low on one scale and fine on another, so a borderline number is best treated as a conversation with your clinician rather than a fixed verdict.
- 03Testing everyone isn't the recommendation
The US Preventive Services Task Force found the evidence insufficient to screen adults who have no symptoms, and in 2024 the Endocrine Society advised against routine screening in healthy people. Testing earns its place when there is a reason, such as a risk factor, a bone problem, or a condition that lowers vitamin D.
Vitamin D deficiency means a 25-hydroxyvitamin D level (25(OH)D, the blood marker that reflects your vitamin D stores over the past few weeks) below 20 ng/mL, which is about 50 nmol/L.2 It is common, and most people who have it feel nothing, so the number can sit under the line for months before any symptom shows up, if one ever does.
If a result comes back under 20 ng/mL, bring it to your clinician. They can look for a cause, advise on how to raise it safely, and arrange a retest. There is no safe generic dose to copy from an article, because the right amount depends on how low you are and why.
What counts as vitamin D deficiency?
Deficiency is defined by the blood number rather than by how you feel. The most widely used line is a 25(OH)D below 20 ng/mL (50 nmol/L), the level the 2011 Endocrine Society guideline set for deficiency, with 21 to 29 ng/mL labelled insufficiency and 30 ng/mL and above called sufficient.2 The Institute of Medicine, reviewing the evidence the same year, concluded that 20 ng/mL already meets the needs of at least 97.5% of people and that deficiency starts below 12 ng/mL (30 nmol/L).3
| 25(OH)D level | Institute of Medicine (2011) | Endocrine Society (2011) |
|---|---|---|
| Below 12 ng/mL (30 nmol/L) | Deficient | Deficient |
| 12 to 20 ng/mL (30 to 50 nmol/L) | At risk of inadequacy | Deficient (below 20) |
| 20 to 30 ng/mL (50 to 75 nmol/L) | Adequate for 97.5% of people | Insufficient (21 to 29) |
| 30 ng/mL (75 nmol/L) and up | No added benefit shown | Sufficient |
That gap is why the same result confuses people. A 25 ng/mL comes back flagged 'insufficient' against the Endocrine Society target and 'adequate' against the Institute of Medicine one. This is the difference between a lab's reference range and an 'optimal' target, covered in reference ranges explained; a borderline number is a reason to talk with your clinician about your own risk, rather than a fixed verdict.
What are the signs and symptoms of low vitamin D?
Most of the time, there are none. Vitamin D deficiency is frequently silent, and the blood level falls before you feel anything.1 For many people the symptoms never arrive at all, which makes them an unreliable way to catch a low level.
When symptoms do show up, they are vague and easy to blame on something else. Tiredness, aching bones or muscles, muscle weakness, and low mood are the ones people most often connect to low vitamin D, and many also ask about more frequent infections. All of these have many causes, and the evidence for vitamin D's effects beyond bone has been inconsistent, so no symptom can confirm deficiency on its own.1 Only a 25(OH)D blood test can.
The symptoms that trace clearly to vitamin D sit at the severe, long-standing end, and they are about bone. When vitamin D stays low for a long time, the body leans harder on parathyroid hormone (PTH, the hormone that manages blood calcium), which pulls calcium out of bone to keep blood levels normal.2 Deep, prolonged deficiency softens the bones, a condition called osteomalacia in adults and rickets in children, causing bone pain, muscle weakness, and a higher risk of fractures.1
Part of why a low level slips by is that it drifts with the seasons.
What causes low vitamin D?
Your body has two supply routes for vitamin D: it makes most of its own in the skin using UVB (the ultraviolet light in summer sunshine), and it absorbs smaller amounts from food and supplements in the gut. Anything that cuts skin production or gut absorption can pull the level down, and the common causes tend to stack together rather than act alone.1
| Cause or risk factor | Why it lowers vitamin D |
|---|---|
| Little sun exposure | Most vitamin D is made in skin from UVB light; mostly-indoor days, covered skin, and sunscreen all reduce it. |
| Darker skin | More melanin absorbs UVB, so the skin makes less vitamin D from the same amount of sun. |
| Higher latitude and winter | Far from the equator, the winter sun is too weak to make vitamin D in skin for months at a time. |
| Older age | Ageing skin converts sunlight to vitamin D less efficiently, and older adults spend more time indoors. |
| Obesity | Vitamin D is fat-soluble and gets held in fat tissue, leaving less circulating in the blood. |
| Malabsorption | Celiac disease, Crohn's, and weight-loss (bariatric) surgery reduce how much vitamin D the gut takes in. |
| Some medicines | Certain anti-seizure drugs and glucocorticoids (steroids) speed up how fast the body breaks vitamin D down. |
These risks add up. A darker-skinned older adult who works indoors through a northern winter can be deficient without anything feeling wrong.12
Who should get tested?
Not everyone. The US Preventive Services Task Force reviewed screening healthy adults who have no symptoms and could not find enough evidence to recommend it either way.4
The USPSTF concludes that the current evidence is insufficient to assess the balance of benefits and harms of screening for vitamin D deficiency in asymptomatic adults.US Preventive Services Task Force, JAMA 2021
The 2024 Endocrine Society guideline went further, advising against routine 25(OH)D testing in healthy adults, and for some groups it now favours giving vitamin D without testing first.5 Testing earns its place when there is a specific reason: symptoms or signs of bone disease, osteoporosis, a condition that limits absorption such as celiac disease or a history of bariatric surgery, certain medicines, or a stack of the risk factors above. If a close relative was found deficient because of a shared cause, such as a gut condition or a mostly-indoor lifestyle, that is worth mentioning to your clinician too.
How the test works, and how often to retest
The test to ask for is 25-hydroxyvitamin D. It needs no fasting and no special preparation, because 25(OH)D reflects your vitamin D over the past few weeks, not your last meal, and it is usually a single blood draw that can be added onto a routine panel.1 There is one common mix-up to avoid when ordering it.
Order 25-hydroxyvitamin D for status, not the active 1,25 form; the active form can read normal while your stores are empty.
The active form, 1,25-dihydroxyvitamin D, is the wrong test for checking your stores; see 25-hydroxyvitamin D for how the assay reads and what the units mean.
Whether a vitamin D test is covered often depends on why it is ordered: a test with a clear medical reason is more likely to be covered than a routine screen with no risk factors, which mirrors the guideline advice against blanket screening. If a level is low and you begin correcting it, your clinician will usually recheck after a few months, because 25(OH)D shifts over weeks rather than days and needs time to settle before a retest means anything.
How is low vitamin D corrected?
The body makes vitamin D from sunlight, and you take in smaller amounts from food, mainly oily fish like salmon and sardines, egg yolks, and fortified milk and cereal, and from supplements.1 When a level is low, clinicians correct it with a course of vitamin D followed by a maintenance amount, matched to how low you are and the cause; someone with obesity or a gut condition that limits absorption often needs a different plan from someone whose main issue is limited sun. More is not better, though: taking high doses over a long time without monitoring can push blood calcium too high, which is another reason the amount is set by your clinician, not by a generic online figure. A retest a few months later confirms it worked.
Link · NIH Office of Dietary SupplementsVitamin D: Fact Sheet for Health ProfessionalsThe reference the ranges, causes, and food sources on this page draw from, with the full evidence on vitamin D and health.ods.od.nih.gov
Citations
- NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals (2024).
- Holick MF, et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2011;96(7):1911-1930.
- Institute of Medicine (US). Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press; 2011.
- US Preventive Services Task Force. Screening for Vitamin D Deficiency in Adults: Recommendation Statement. JAMA. 2021;325(14):1436-1442.
- Demay MB, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.