A vitamin D level below 20 ng/mL is generally considered low, and anything below 12 ng/mL is classified as deficient. These numbers come from a standard blood test called the 25-hydroxy vitamin D test, which measures the total amount of vitamin D circulating in your body. Your results will typically appear in nanograms per milliliter (ng/mL), and where you fall on that scale determines whether your levels need attention.
How to Read Your Results
The NIH uses these thresholds for interpreting vitamin D blood levels:
- Below 12 ng/mL: Deficient. Associated with bone disease in both children and adults.
- 12 to 19 ng/mL: Inadequate for bone and overall health.
- 20 ng/mL and above: Generally adequate for most healthy people.
- Above 50 ng/mL: Potentially harmful, with risk of toxicity increasing above 60 ng/mL.
There is some disagreement among experts about the “adequate” cutoff. Some practitioners consider 30 ng/mL the minimum target and recommend a range of 30 to 50 ng/mL, while others accept 20 ng/mL as sufficient. Lab reports may flag your result differently depending on which standard that lab uses. The important number is your total 25-hydroxy vitamin D level, not any of the individual subtypes that sometimes appear on the report.
Why Vitamin D Levels Matter for Your Bones
Vitamin D’s primary job is helping your intestines absorb calcium from food. Without enough vitamin D, your body can only absorb a fraction of the calcium you eat, no matter how much dairy or calcium-rich food you consume. When calcium absorption falls short, your body compensates by pulling calcium out of your bones to keep blood calcium levels stable. Over time, this leads to weaker, less dense bones.
In adults, severe deficiency causes a condition called osteomalacia, which results in soft, weak bones, persistent bone pain, and muscle weakness. In children, the same process produces rickets, where growing bones fail to harden properly and growth plates enlarge. Even levels that are merely “inadequate” rather than fully deficient can contribute to gradual bone density loss and raise the risk of fractures, particularly as you age.
Symptoms of Low Vitamin D
Low vitamin D often produces no obvious symptoms for years, which is part of why it goes undetected so frequently. When symptoms do appear, they tend to be vague enough that people attribute them to aging, stress, or poor sleep. Bone pain, particularly in the lower back, hips, or legs, is one of the more specific signs. Muscle weakness is another, sometimes showing up as difficulty climbing stairs or getting up from a chair.
Chronic deficiency has also been linked to broader health risks beyond the skeleton. Research has connected persistently low vitamin D to increased risk of high blood pressure, cardiovascular disease, type 1 diabetes, multiple sclerosis, and rheumatoid arthritis. The strength of these associations varies, and having low vitamin D does not mean you will develop these conditions. But it does suggest that vitamin D plays a wider role in immune function and inflammation than its reputation as a “bone vitamin” implies.
Who Is Most Likely to Be Low
Your body produces vitamin D when ultraviolet B rays from sunlight hit your skin. Anything that reduces that exposure reduces your vitamin D production. Geography is a major factor: if you live above about 40 degrees north latitude (roughly the line running through Boston, Madrid, or Beijing), there is not enough UVB radiation from roughly November through early March to produce meaningful vitamin D. Farther north, in places like Edmonton or Scandinavia, that “vitamin D winter” stretches from October through April.
Skin color matters significantly. Melanin, the pigment that makes skin darker, competes with the vitamin D precursor in your skin for UV absorption. People with dark skin may need up to ten times longer in the sun to produce the same amount of vitamin D as someone with fair skin. Age also reduces production capacity because older adults have lower concentrations of the precursor compound in their skin. People who spend most of their day indoors, wear clothing that covers most of their body, or consistently use sunscreen are also at higher risk.
Obesity is another independent risk factor. Vitamin D is fat-soluble, meaning it gets sequestered in fat tissue rather than remaining available in the bloodstream. People with higher body fat percentages often need more vitamin D to maintain adequate blood levels.
How Low Vitamin D Is Corrected
If your levels are severely low, your doctor will typically prescribe a loading dose, a short course of high-dose vitamin D taken over several weeks to bring your levels up quickly. One common approach is 50,000 IU once a week for six weeks. After that initial correction, a maintenance dose of 800 to 2,000 IU daily keeps levels stable.
For people who are not deficient but simply want to maintain adequate levels, 400 IU daily is a standard dose for adults. Children prescribed supplementation for deficiency take lower loading doses adjusted by age, followed by 400 to 600 IU daily for long-term maintenance. Vitamin D3 (the form your skin naturally produces) is generally preferred over D2, and taking it with a meal that contains some fat improves absorption.
It is worth noting that more is not better. Blood levels above 150 ng/mL are associated with toxicity, which causes dangerously elevated calcium in the blood. This essentially never happens from sun exposure or normal food intake. It results from taking very high supplement doses without medical guidance over extended periods.
Should You Get Tested?
The Endocrine Society’s 2024 guidelines recommend against routine vitamin D testing for most healthy adults, including those with darker skin or obesity. This is not because deficiency is rare, but because the evidence suggests that for most people, the benefit of empiric supplementation (just taking a standard dose without testing first) is comparable to testing and then treating. Routine testing adds cost without clearly improving outcomes for the average person.
The guidelines do recommend supplementation without testing for certain groups: children and adolescents ages 1 to 18, adults 75 and older (where it may reduce mortality risk), pregnant women (where it may lower risk of preeclampsia and preterm birth), and adults with prediabetes at high risk for developing type 2 diabetes. For healthy adults under 75 without specific risk factors, the guidelines suggest supplementation is generally unnecessary.
If you already have a test result in hand, the thresholds above give you a clear framework. A result below 12 ng/mL calls for active treatment. A result between 12 and 20 ng/mL is worth addressing with supplementation. And a result above 20 ng/mL means your levels are, for most purposes, where they need to be.