Vitamin D Testing: Who Needs It and What the Results Mean

You may have heard that many people have low vitamin D, and you want to know if you should be tested. Or your result came back "insufficient" and you are not sure what that means. This guide explains what the test measures, how to read the numbers, and why most guidelines do not recommend testing everyone.

What vitamin D does

Vitamin D helps your body absorb calcium and keep bones strong. Your skin makes vitamin D in sunlight. You also get it from fortified milk and plant drinks, fatty fish, egg yolks, and supplements [1].

Very low vitamin D over a long time can cause soft, weak bones. This is called osteomalacia in adults and rickets in children. Low levels have also been linked to many other conditions in observational studies, but trials have not shown that supplements prevent most of them [3].

What the test measures

The standard test measures 25-hydroxyvitamin D, written 25(OH)D. This is the main form stored in your blood. It reflects vitamin D from sun, food, and supplements over the past few weeks [1].

Another test, 1,25-dihydroxyvitamin D, measures the active hormone. It is not a good test for deficiency and is used only in special cases, such as kidney disease or calcium problems.

Units

• Canadian labs usually report nmol/L.
• Many US labs report ng/mL.
• To convert, divide nmol/L by 2.5. So 50 nmol/L equals 20 ng/mL.

What your doctor looks at

The reference values

Canada and the US share the same Dietary Reference Intakes for vitamin D, set by the US National Academies (formerly the Institute of Medicine) and adopted by Health Canada. Under these values:

• Below about 30 nmol/L (12 ng/mL): risk of deficiency and bone problems.
• 30 to 50 nmol/L (12 to 20 ng/mL): may be inadequate for some people.
• About 50 nmol/L (20 ng/mL): enough for bone health in most people.
• Above about 125 nmol/L (50 ng/mL): no added benefit has been shown, and very high levels may cause harm.

Some labs and some specialist groups use 75 nmol/L (30 ng/mL) as the "sufficient" cut point. This is why the same result can be called "normal" by one lab and "insufficient" by another. The 2024 Endocrine Society guideline chose not to endorse a specific target level for disease prevention, because trials did not show clear benefits from aiming for higher levels [3].

Who should be tested

Major guidance in both countries says routine testing of healthy adults is not useful.

• The US Preventive Services Task Force (USPSTF) found insufficient evidence to recommend for or against screening adults without symptoms (an "I" statement) [2].
• The Endocrine Society found no trial evidence to support routine 25(OH)D screening in the general population [3].
• Choosing Wisely Canada advises against vitamin D testing in low-risk adults [4].

Testing is more reasonable when results could change care. Examples include:

• Osteoporosis, low bone density, or repeated fractures.
• Conditions that reduce absorption, such as celiac disease, inflammatory bowel disease, or weight loss surgery.
• Chronic kidney or liver disease.
• Long-term use of some seizure medicines or steroid medicines.
• Abnormal calcium or parathyroid hormone levels.
• Symptoms that suggest bone softening, such as bone pain and muscle weakness.

Who may benefit from vitamin D without testing

The 2024 Endocrine Society guideline suggests vitamin D above the usual daily intake, without testing first, for some groups [3]:

• Children and teens aged 1 to 18.
• Adults aged 75 and older.
• People who are pregnant.
• Adults with high-risk prediabetes.

For healthy adults under 75, it suggests against taking more than the daily recommended intake to prevent disease [3].

Why one result can mislead

A single vitamin D level is a snapshot. Levels are usually lower at the end of winter and higher in late summer, especially in Canada and the northern US. Different lab methods can also give slightly different results for the same blood sample [1]. If your level is close to a cut point, the season and the lab method may matter more than a small change in the number. This is one reason clinicians focus on whether a result will change what they do.

Daily intake

The recommended daily intake in Canada and the US is 600 IU for most people aged 1 to 70 and 800 IU for people over 70. The tolerable upper limit for adults is 4,000 IU per day. Do not take high doses without advice from your clinician or pharmacist, because too much vitamin D can raise calcium to unsafe levels.

Questions to ask your clinician

• Is a vitamin D test likely to change my care?
• Which cut point does my lab use, and what does my result mean?
• Do I have a condition or take a medicine that affects vitamin D?
• How much vitamin D should I get each day from food and supplements?
• Do I need a bone density test as well?
• Should the test be repeated, and when?

When to get care quickly

Very high vitamin D from large supplement doses can cause high calcium. Get care quickly if you have severe vomiting, confusion, severe weakness, strong thirst, or passing a lot of urine, especially if you have been taking high-dose vitamin D. Call 911 for confusion or fainting.

Bottom line

The vitamin D test measures your stored vitamin D. Most people are fine at about 50 nmol/L (20 ng/mL). Canadian and US guidance does not support routine testing for healthy adults, but testing can help when you have bone disease, absorption problems, or certain medicines. For most people, meeting the daily intake is more useful than chasing a number.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.