Vitamin D Deficiency and Testing: What Blood Levels Mean

Overview

A vitamin D blood test usually measures a form called 25-hydroxyvitamin D, written 25(OH)D. MedlinePlus calls this the most accurate way to see whether you have enough vitamin D. [1] The harder questions are who needs the test, what number counts as "enough", and what to do with the result. Expert groups answer these differently, and this page shows where they disagree instead of hiding it. Background on the vitamin itself is on Vitamin D. This is information, not a way to diagnose yourself.

What deficiency means

MedlinePlus describes vitamin D deficiency as very low levels that can lead to bone disorders and other medical conditions. [1] Long-lasting deficiency can weaken bones: osteomalacia (soft bones) and osteoporosis in adults, and rickets in children. [2] The NHS (UK) explains the same link and says a lack of vitamin D can lead to bone deformities in children and bone pain in adults. [3] More detail is on Rickets and Osteomalacia.

Who is more likely to be low

MedlinePlus (US) lists people who rarely expose their skin to sunlight, people with malnutrition, people who have had weight-loss surgery, people with conditions such as Crohn's disease, ulcerative colitis or celiac disease, people with kidney or liver disease, and people taking certain medicines such as some statins, steroids and weight-loss medicines. [1] Babies fed mainly breast milk are also listed, because breast milk is low in vitamin D. [1] The NHS adds that people with dark skin may not make enough vitamin D from sunlight. [3]

Symptoms cannot confirm deficiency

Bone pain, muscle weakness or aches, soft or deformed bones and fractures can be related to low vitamin D, and a clinician may order a test when they appear. [1] They are not proof of it, and MedlinePlus notes that low vitamin D can weaken bones even when a person has no symptoms. [1] Do not diagnose yourself from a symptom list. The broader picture is on Vitamin Deficiency Testing.

Reading the number: units and cut-offs

Results are given in nanomoles per liter (nmol/L) or nanograms per milliliter (ng/mL), and one MedlinePlus page gives the conversion as 0.4 ng/mL = 1 nmol/L. [2] Reports may show D2 and D3 separately or as "total vitamin D", and MedlinePlus says total vitamin D is the important number. [1]

These figures do not match, so a level flagged "insufficient" on one report may be called adequate by another source. Treat the flag as a reason to talk with your clinician, not as a diagnosis.

Why results differ between labs

An Australian study compared assays on samples from 813 people. At one laboratory, an immunoassay classified 46% of 765 participants as deficient (below 50 nmol/L) against 17% by mass spectrometry, the reference method, and the authors concluded that between 1 in 5 and 1 in 3 people could be misclassified as deficient. [7] This study dates from 2012 and assays have changed since, so it shows that one number deserves caution rather than describing today's labs. MedlinePlus also says normal ranges vary slightly among labs. [4]

Who should be tested?

• MedlinePlus says routine vitamin D testing is not recommended for everyone, and screening all adults, even in pregnancy, is generally not recommended. [1][4]
• Testing may be done in people at high risk, such as those over 65, with obesity or after weight-loss surgery, taking certain medicines, with thin bones, limited sun exposure or absorption problems. [4]
• The US Preventive Services Task Force found the evidence insufficient to judge the benefits and harms of screening adults who have no symptoms. [5]
• The Endocrine Society (2024) found no trial evidence for routine testing in the general population, including people with obesity or dark skin, and suggests against routine testing where there is no established reason. [6] It says this does not apply to people with established indications for testing or treatment. [6]
• A test can also check whether levels are improving in someone treated for deficiency, or look for too much in someone with symptoms after taking supplements. [1]

Official intake reference points

These are published levels for stated groups, not advice for any one person. A clinician decides what is right for you.

Both sources give the unit conversion as 1 microgram = 40 IU. [3][2] For how the figures are defined see RDA, AI, DV and UL, and for more on units see Vitamin D Intake Levels.

Related reading

Osteoporosis and bone health · Vitamin D2 vs D3 · Deficiency Testing and Its Limits