Vitamin D deficiency: who should be tested?

In short

• Vitamin D helps the body absorb calcium, which bones need, and it also plays a role in muscles and the immune system.
• Mild deficiency often causes no clear symptoms, so you cannot diagnose it from how you feel.
• Routine testing is not recommended for everyone. A blood test is used for people with risk factors, symptoms or bone problems.
• The body makes vitamin D from sunlight, and food and supplements add to it.
• Too much vitamin D from supplements can raise blood calcium and harm the kidneys.

What vitamin D does

Vitamin D helps the body absorb calcium, which bones depend on, and it also plays a role in the nervous, muscle and immune systems. You get it from sunlight on your skin, from food and from supplements.

When levels stay low for a long time, bones lose density, which can contribute to osteoporosis and fractures. Severe deficiency causes rickets in children, which softens and bends the bones, and osteomalacia in adults, which causes weak bones, bone pain and muscle weakness.

Symptoms of low vitamin D

• Bone pain.
• Muscle weakness or cramps.
• Fatigue and mood changes such as low mood.
• In children, bowed or bent bones and joint deformities.
• No symptoms at all, which is common.

Tiredness, aches and low mood have many other causes, so feeling tired is not a good reason to assume you are low on vitamin D. A clinician looks at the whole picture.

Who is more likely to be low

• Adults over 65 or people who are homebound or rarely outdoors.
• People with darker skin.
• People who cover most of their skin with clothing when outside.
• People with obesity or who have had gastric bypass or other weight-loss surgery.
• People with Crohn's disease, ulcerative colitis, celiac disease, cystic fibrosis, or kidney or liver disease.
• Infants who are only breastfed.
• People taking certain medicines, including some for seizures, steroids, some cholesterol and weight-loss medicines.

Low intake, poor absorption, limited sun exposure and kidney or liver problems that stop the body from converting vitamin D to its active form are the main causes.

When testing makes sense

The usual test is a blood test for 25-hydroxyvitamin D, written 25(OH)D. MedlinePlus says routine vitamin D testing is not recommended for everyone, and the US Preventive Services Task Force found the evidence insufficient to judge screening for people without symptoms.

Providers order it when a bone problem or another condition may be linked to low vitamin D, when you have symptoms such as bone pain, muscle weakness or fractures, and for people at high risk, for example with limited sun exposure or malabsorption conditions. It can also check whether treatment is working. No special preparation is needed, but tell your provider about every medicine and supplement you take.

Results are reported as deficient, insufficient, sufficient or optimal, or toxic. What a number means for you is a question for your clinician, not for a table online.

Sun and food

Sunlight is a source, but skin tone, age and where you live limit how much your body makes. Spending long in the sun without sunscreen raises skin cancer risk, so tanning for vitamin D is not a good plan. You cannot overdose from sunlight, but you should still protect your skin.

• Oily fish such as salmon, sardines and mackerel.
• Egg yolks.
• Red meat and liver, though liver should be avoided in pregnancy.
• Fortified foods such as some cereals and spreads.

Supplements and treatment in general terms

Treatment for deficiency is usually a supplement, and the amount, frequency and length of time are set by your provider. The goal is to reach and then maintain an adequate level.

• There are two forms. D2 comes from plants, and D3 comes from animals and is absorbed more easily.
• Tell your clinician about every supplement, including multivitamins, since vitamin D can be in several of them.
• A clinician decides when a repeat blood test is needed.
• Taking vitamin D only to prevent COVID-19 is not supported, according to the NHS, because the evidence is not strong enough.

Babies and children

Infants who are only breastfed are in a higher-risk group, and severe deficiency in a child can cause rickets, with bowed legs, bone pain and muscle weakness. Ask your pediatrician whether your baby needs vitamin D, and let the pediatrician choose the product and the amount. Children are not small adults when it comes to supplements, so avoid giving them anything made for grown-ups.

The risks of taking too much

Toxicity is rare and usually comes from excess supplements, not from sun or food. Too much can raise blood calcium, called hypercalcemia, with nausea, vomiting, thirst, constipation, weakness, weight loss and confusion. Over time it can weaken bones and damage the kidneys and heart.

Large amounts of vitamin A, which can appear in some fish oils, can also be harmful. Do not take high amounts because of something you read online.

When to see a doctor, and red flags

Book a visit for persistent bone pain, muscle weakness, fractures from minor injuries, or in a child, bowed legs. Raise it too if you have bowel, liver or kidney disease or take medicines linked with low vitamin D. Bring a list of every supplement you take, including the label.

Call 911 for severe confusion, fainting or an irregular heartbeat if you have been taking large amounts of vitamin D.

Questions and answers

Sources

1. MedlinePlus — Vitamin D Deficiency
2. MedlinePlus — Vitamin D Test
3. NHS — Vitamin D
4. Cleveland Clinic — Vitamin D Deficiency
5. USPSTF — Vitamin D Deficiency in Adults: Screening

Sources checked on 29 October 2026.