Vitamins & Hormones
What does low vitamin D mean?
The short answer
A vitamin D blood test measures 25-hydroxyvitamin D, the main circulating form and the most accurate guide to whether you have enough. A low result usually means too little sunlight, food or supplement intake, and a long-term shortfall can weaken bones and muscles. US national guidance treats below 20 ng/mL as inadequate for most people, though many labs flag anything below 30 ng/mL.
What Vitamin D measures
Vitamin D helps your body absorb calcium and keep bones and muscles strong. The body converts vitamin D from sunlight, food and supplements into 25-hydroxyvitamin D, or 25(OH)D, which is what the test measures. Many reports call it "Vitamin D Total": D2 and D3 added together.
Sunlight on bare skin is the main source for most people. Living where sunlight is limited, cloudy weather, shade and darker skin all reduce how much vitamin D the skin makes.
Vitamin D ranges
Two sets of US cut-offs are in common use, the National Academy of Medicine thresholds published by NIH and the higher cut-offs many US labs print, and the Endocrine Society's 2024 guideline found no clear evidence for an optimal target level.
| Result | Range |
|---|---|
| Deficiency risk (National Academy of Medicine, via NIH)Linked to osteomalacia (soft bones) in adults | Below 12 ng/mL |
| Inadequate (National Academy of Medicine, via NIH) | 12 to under 20 ng/mL |
| Adequate (National Academy of Medicine, via NIH)Enough for most people | 20 ng/mL and above |
| Deficient (many US labs) | Below 20 ng/mL |
| Insufficient (many US labs) | 20 to 29 ng/mL |
| Sufficient (many US labs) | 30 to 100 ng/mL |
| Potential adverse effects (National Academy of Medicine, via NIH) | Above 50 ng/mL |
| Possible toxicity (many US labs)Speak to your doctor | Above 100 ng/mL |
A result of 25 ng/mL is adequate by National Academy of Medicine thresholds but insufficient for labs using a 30 ng/mL cut-off. Ranges vary between laboratories, and the range printed on your own report takes priority. Ranges vary between laboratories, and the range printed on your own report takes priority.
What a high vitamin D result means
A high result almost always comes from supplements. The skin limits how much vitamin D it makes, so sunlight cannot cause an overdose. Too much vitamin D makes the gut absorb too much calcium, and high blood calcium (hypercalcaemia) can cause nausea, vomiting, constipation, poor appetite, weakness, confusion, kidney stones and kidney damage.
Common causes
- Long-term high-dose supplements
- Several vitamin D products taken together, such as a multivitamin plus vitamin D
- High-dose courses prescribed to treat a deficiency
- Not sunlight, which cannot cause toxicity
What a low vitamin D result means
A low result means less vitamin D than your body needs to absorb calcium well. Mild shortfalls often cause no symptoms. Long-term deficiency can cause bone pain, muscle weakness and, in adults, soft bones (osteomalacia) and fractures. Whether you count as low depends on your lab's cut-off.
Common causes
- Little sun on your skin, from staying indoors, being homebound or living where sunlight is limited
- Darker skin, which makes less vitamin D from sunlight
- Age over 65, as the skin makes less vitamin D with age
- Obesity
- Crohn's disease, ulcerative colitis, coeliac disease or weight loss surgery
- Kidney or liver disease
- Some medicines, including corticosteroids and anti-seizure drugs such as phenytoin
What moves Vitamin D
Nutrition
Fatty fish such as salmon, tuna and mackerel are among the best food sources. Egg yolks, beef liver and cheese provide small amounts, and most US milk and many breakfast cereals are fortified. Whether you need a supplement, and at what dose, is your doctor's call.
Training
Outdoor training adds sun exposure, which is how the skin makes vitamin D. MedlinePlus advises against more than a few minutes of unprotected sun because of skin cancer risk, so balance the two.
Sleep and stress
No strong evidence links sleep or stress directly to vitamin D levels, so this is not a main lever for this marker.
Habits
If your doctor recommends a supplement, take it as advised and keep a list of every product you use that contains vitamin D, so you do not double up.
How fast Vitamin D can change
Vitamin D moves over weeks, not days. Testing.com advises retesting 8 to 12 weeks after starting a supplement, so a Day 90 re-test is a meaningful window. Sun exposure also changes through the year, so a summer result and a winter result can differ with no change in habits.
Before your test
- Note the season of each test. Sun exposure changes through the year, so results from different seasons are not a like-for-like comparison.
- Some labs recommend fasting but not all require it. Follow the instructions with your test.
- Tell your doctor about all supplements and medicines. Supplements raise the result, and corticosteroids and anti-seizure drugs such as phenytoin can lower it.
- Labs use different cut-offs, so results from two labs are not always comparable.
When to speak to your doctor
Speak to your doctor if your result is below your lab's range, especially with bone pain, muscle weakness or a past fracture. Contact them promptly if it is above 100 ng/mL, or if you take vitamin D and develop nausea, vomiting, constipation or muscle weakness, which can be signs of high calcium. Confusion, or vomiting that will not stop, needs same-day medical care. If you take a thiazide diuretic (water pill), tell your doctor about any vitamin D supplement, as the combination can lead to high calcium. Do not change a supplement dose without your doctor.
On the Wolfburgh panel
Vitamin D (25-Hydroxy) is one of the 17 markers on the Wolfburgh blood panel. Your sample is collected at home with RedDrop One, an FDA-cleared collection device, and a licensed physician reviews every panel before your care team sees it.
The partner lab reports vitamin D as Vitamin D Total in ng/mL. The Nutritionist works on food, the Trainer on outdoor activity and the Behaviour Specialist on routine, in one shared plan with the Partner Physician. Members re-test at Day 90, backed by the 90-Day Results Guarantee.
Common questions about Vitamin D
What is a normal vitamin D level?
It depends on which cut-off your lab uses. US national guidance (the National Academy of Medicine, published by NIH) treats 20 ng/mL or more as adequate for most people and below 12 ng/mL as a risk of deficiency. Many US labs use higher cut-offs, calling 30 to 100 ng/mL sufficient and 20 to 29 ng/mL insufficient. The range printed on your own lab report takes priority.
How can I raise my vitamin D level?
Sunlight on bare skin is the main source for most people, though MedlinePlus advises against more than a few minutes of unprotected sun because of skin cancer risk. Fatty fish such as salmon and mackerel are among the best foods, and most US milk and many cereals are fortified. If your level is low, your doctor will advise whether you need a supplement and at what dose.
Can you have too much vitamin D?
Yes, almost always from supplements rather than sunlight. Too much vitamin D raises blood calcium (hypercalcaemia), which can cause nausea, vomiting, weakness, confusion, kidney stones and kidney damage. NIH links levels above 50 ng/mL with potential adverse effects, and many US labs flag results above 100 ng/mL as possible toxicity. Speak to your doctor about any high result.
Why do labs disagree on what counts as low vitamin D?
Because the evidence does not settle it. The National Academy of Medicine, whose figures NIH publishes, treats 20 ng/mL or more as adequate for most people, while many US labs flag anything below 30 ng/mL. The Endocrine Society's 2024 guideline found no clear evidence for one optimal target and suggests against routine testing in healthy people without a specific reason.
Sources
- NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals
- Testing.com. Vitamin D Test
- Journal of Clinical Endocrinology and Metabolism (PubMed). Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline (Demay et al., 2024)
- MedlinePlus (NIH). Vitamin D Test
- MedlinePlus (NIH). Vitamin D
- Cleveland Clinic. Vitamin D Deficiency
This page is general information, not medical advice, and it does not replace a consultation with your own doctor. Never start, stop or change a medicine or supplement because of something you read here.
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