Vitamin D test (25-hydroxy): what it measures, normal levels, and who should be tested

Vitamin D arrives two ways: your skin makes vitamin D3 from UVB sunlight, and you take in D3 from animal foods and most supplements and D2 from fortified foods, some mushrooms and prescription D2. The liver converts both into 25-hydroxyvitamin D, which circulates for two to three weeks and is the form the body draws on. The kidneys then activate a small amount into 1,25-dihydroxyvitamin D, the hormone that actually raises calcium absorption.

The standard test measures total 25-hydroxyvitamin D (D2 plus D3). It is the right test for status because it has a long half-life and tracks intake; 1,25-dihydroxyvitamin D is tightly regulated, stays normal or even rises in deficiency, and is only ordered for specific kidney, parathyroid and granulomatous conditions. Results are reported in nanograms per milliliter (ng/mL) in the US and nanomoles per liter (nmol/L) elsewhere; multiply ng/mL by 2.5 to convert.

Most people with a low 25-hydroxyvitamin D have no symptoms at all, which is why deficiency is usually found on a blood test rather than by how someone feels. When symptoms do appear they are vague and overlap with many other conditions:

Too much vitamin D from food or sunlight is essentially unheard of; toxicity comes from high-dose supplements taken for weeks to months and acts through a high blood calcium — nausea, vomiting, constipation, excessive thirst and urination, confusion, weakness and kidney stones.

Because none of these symptoms is specific, the test rather than the symptom list settles the question. A result in the deficient band in someone with symptoms is a reason to discuss repletion with a clinician, and a recheck after about three months shows whether the level has moved.

The Endocrine Society's 2024 guideline recommends against routine vitamin D testing in generally healthy adults, because the evidence does not show that testing and treating people without risk factors improves outcomes. Testing is aimed instead at people whose result would change what they do: those with osteoporosis or an unexplained fracture, bone pain or muscle weakness, or a condition that impairs absorption or metabolism.

The groups most likely to have a low level — and the usual reasons a clinician orders the test — are people with celiac disease, Crohn's disease, cystic fibrosis or previous bariatric surgery; chronic kidney or liver disease; obesity; darker skin; very limited sun exposure from indoor work, covering clothing or northern latitude; older adults, especially with falls or fractures; people on anticonvulsants, glucocorticoids, certain HIV medications or long-term antifungal therapy; and anyone with an abnormal calcium, phosphorus, alkaline phosphatase or parathyroid hormone result.

The other common reason to test is follow-up: confirming that a previously low level has responded after about three months of supplementation, or checking that a high-dose regimen has not overshot.

The test is a routine blood draw with no fasting and no time-of-day requirement. You can order it online through TestWell — a licensed physician reviews the order, you visit a nearby Quest or Labcorp patient service center, and results arrive in your portal, typically within a few business days — or have your own clinician order it. It is commonly bundled into wellness, vitamin-deficiency and bone-health panels.

Two things affect interpretation. Season: levels bottom out in late winter and peak in late summer, so a February result is close to your personal low and an August result close to your high. Supplements: 25-hydroxyvitamin D takes roughly three months to plateau after a change in dose, so testing a few weeks into a new supplement shows a level still climbing. If you have recently taken a very large single dose, mention it, because it can raise the level temporarily. Ordinary daily supplements do not need to be stopped before the draw.

Two expert bodies have drawn the lines in slightly different places, which explains why you may see 20 ng/mL in one article and 30 ng/mL in another. The Institute of Medicine (now the National Academies) concluded in 2011 that 20 ng/mL (50 nmol/L) covers the bone-health needs of at least 97.5% of the population. The Endocrine Society's 2011 clinical guideline set deficiency below 20 ng/mL, insufficiency at 21–29 ng/mL and sufficiency at 30 ng/mL and above, and most US laboratories, including Quest and Labcorp, print 30–100 ng/mL as the reference range.

Below about 12 ng/mL the risk of rickets in children and osteomalacia in adults rises clearly, and that is where nearly everyone agrees treatment is needed. Levels above 100 ng/mL are flagged as potentially toxic, though symptoms of toxicity — driven by high calcium — usually appear well above 150 ng/mL and almost always come from very high supplement doses rather than sun or food.

Sunlight is the largest input for most people, and everything that limits UVB — latitude, season, time indoors, clothing, sunscreen, air pollution and darker skin, which needs longer exposure to make the same amount — lowers the level. Diet contributes less than people expect; few foods contain much vitamin D naturally, and fortified milk, cereals and fatty fish are the main sources.

Body composition matters: vitamin D is stored in fat, so people with obesity have lower circulating levels for the same intake and often need more to reach a target. Fat malabsorption from celiac disease, Crohn's disease, pancreatic insufficiency or bariatric surgery reduces absorption from food and supplements alike. Kidney and liver disease impair the conversion steps, and medications that speed up liver metabolism — some anticonvulsants, glucocorticoids and rifampin — break down vitamin D faster. Genetic differences in the vitamin D binding protein also shift measured totals between individuals and ancestries.

Vitamin D does not act alone, and a low or high result is usually interpreted with calcium, phosphorus, alkaline phosphatase and parathyroid hormone (PTH). Long-standing deficiency raises PTH as the body fights to hold calcium steady; a high vitamin D with a high calcium suggests over-supplementation; a high calcium with a low PTH and a normal 25-hydroxy level points elsewhere. Kidney function (creatinine, eGFR) matters because the kidneys perform the final activation step.

The 1,25-dihydroxyvitamin D test is reserved for specific situations — suspected sarcoidosis or other granulomatous disease, certain lymphomas, inherited disorders of vitamin D metabolism and advanced kidney disease — and is not a substitute for the 25-hydroxy test when the question is simply whether your stores are adequate.

A low result usually leads to supplementation, with the dose and form chosen by a clinician based on how low the level is, body weight, absorption and other conditions — it is not one-size-fits-all, and very high doses carry their own risks. Because the level takes about three months to plateau, a follow-up test around then confirms the response; once in range, many people recheck yearly, often in late winter.

A high result prompts a review of every supplement and fortified product you take and a calcium check; the interpretation sections below go through the causes, symptoms and next steps for both directions. Bone density testing, a celiac screen or a broader malabsorption work-up follows when the low level is severe, recurrent or unexplained.