Vitamin Blood Tests: Are They Necessary, and Which Ones Matter?

Lately there are plenty of packages testing 20–30 vitamins and minerals to choose from. Many patients bring me their results with the question, "This one is only slightly low. Do I need to take something?" Most of the time the answer is no, and sometimes the real problem isn't in those results at all. This article will help you know which tests are worth it and which aren't necessary.

Why testing every vitamin rarely helps

• Normal ranges are wide and results are hard to interpret. For many vitamins, blood levels don't reflect what is stored in the body, and they shift with your last meal, inflammation or pregnancy.
• A slightly low value is not the same as a deficiency, but it makes many people anxious and leads them to buy too many supplements.
• It doesn't change treatment. If a result won't make us do anything differently, the test is usually not worth it.
• The Endocrine Society's 2024 guideline does not recommend routine vitamin D testing in healthy adults without a specific reason.

Tests that are genuinely useful when there is a reason

Vitamin B12

• Long-term metformin or PPI antacids
• Vegan diet, older age
• Numb hands or feet, pallor, worsening memory

Iron / ferritin

• Pallor, fatigue, hair loss
• Chronic bleeding, or suspected iron overload

Folate

• Macrocytic anemia
• Heavy alcohol use

Vitamin D

• Malabsorption, kidney or liver disease
• Osteoporosis, easy fractures
• Almost no sun exposure

Calcium, phosphorus

• Suspected bone or parathyroid disease
• Taking high-dose vitamin D

Vitamins that build up in the body are more dangerous in excess

Fat-soluble vitamins are stored in the liver and fat and are hard to clear, so high doses taken for a long time can become toxic.

1. Vitamin A in high doses is toxic to the liver, causes headaches and thinner bones, and can cause birth defects if taken during pregnancy. In smokers, high-dose beta-carotene supplements increased lung cancer risk in the ATBC and CARET studies.
2. Vitamin D in excess causes high blood calcium, nausea, confusion, kidney stones and kidney damage. This genuinely happens in people who take very high doses on their own without monitoring.
3. Vitamin E in high doses increases bleeding risk, especially in people taking blood thinners, and the SELECT study found that 400 IU of vitamin E a day increased prostate cancer risk in men.
4. Vitamin K is rarely toxic but affects the blood thinner warfarin. People taking warfarin should not suddenly change supplements or start eating large amounts of green leafy vegetables.

Some water-soluble vitamins can also be overdone, such as vitamin B6, found in multivitamins, energy drinks and many anti-stress supplements. Combining several products over a long time can cause nerve inflammation with numb hands and feet; Australia's medicines regulator issued warnings about this in 2023–2024. High-dose vitamin C raises the risk of kidney stones in men.

Something people often don't know: biotin can skew blood tests

High-dose biotin (vitamin B7) in hair, skin and nail supplements interferes with many laboratory test methods, making thyroid, sex hormone and even cardiac enzyme (troponin) results differ from the truth. The US Food and Drug Administration has issued a warning about this. If you are about to have hormone tests, for example before starting TRT or gender-affirming hormones, tell your doctor; you usually need to stop biotin for at least 2–3 days before the blood draw.

What I recommend

• Start with food, sleep and exercise before thinking about supplements.
• Bring every supplement you take for your doctor to see (a photo of the label is fine). Many people get the same vitamin from several products.
• Test only what there is a reason for, and repeat a test only when the result will change your care.
• If you are taking high-dose vitamin D prescribed by a doctor, there should be a follow-up plan.

Read on in Calcium, zinc and vitamin D for middle-aged men and Health checkups for working-age adults: what to test.

1. 1Basics firstFood, sleep, exercise
2. 2Bring the labelsEvery product you take; many people double up on vitamins
3. 3Test only with a reasonRetest only when the result would change your care
4. 4Have a follow-up planIf your doctor prescribes high-dose vitamin D

High-dose biotin skews blood results; usually stop it 2–3 days before hormone tests.

When to see a doctor

See a doctor if you have numb hands or feet, anemia, get tired easily, have worsening memory, unusual hair loss or bones that break easily, or if you take several supplements and feel nauseous, urinate often or feel confused. At Dr. Kris Clinic I help choose the tests you actually need based on your symptoms and risks, explain the results clearly, and plan what to take or stop. English-speaking consultations are available.