25-hydroxyvitamin D · 25(OH)D
Vitamin D: what your result means
The test measures the vitamin D stored in your body, most of which your skin made from sunlight. Across most of Europe and the northern United States, a low result in winter is the rule rather than the exception.
What it measures
Laboratories measure 25-hydroxyvitamin D, the form the liver makes from what your skin produces and what you eat. It is the best indicator of status because it lasts weeks in the blood — the active form lasts hours, is tightly regulated, and says almost nothing about your stores.
Why it gets tested
For bone pain, muscle weakness, falls or fractures, osteoporosis and malabsorption, and in people with little sun exposure, darker skin at high latitudes, or on medication that depletes it. Testing everybody is generally not recommended: this is a test for people with a reason, not a routine addition.
Typical ranges
There is genuine disagreement here, so the labels differ more than the numbers. Below 25–30 nmol/L (10–12 ng/mL) is widely agreed to be deficiency; 30–50 nmol/L (12–20 ng/mL) is usually called insufficiency; above 50 nmol/L (20 ng/mL) is considered adequate for bone health by most bodies, while some prefer 75 nmol/L (30 ng/mL). Which set your laboratory prints changes the label on an identical result.
Compare against the range printed on your own report, not this one. Laboratories differ, and yours is the one your doctor is reading.
What a high result can mean
Almost always supplements, and this is one of the few vitamins where too much causes real harm — very high levels raise blood calcium, which affects the kidneys and heart rhythm. It takes sustained large doses, not an ordinary daily supplement. Rarely it reflects granulomatous disease such as sarcoidosis, or some lymphomas.
What a low result can mean
Not enough sunlight or intake, which at northern latitudes in winter covers most of the population. It is also lower with darker skin, in older age, with obesity (it distributes into fat), in malabsorption — coeliac disease, Crohn's, after bariatric surgery — and with anticonvulsants and glucocorticoids. Severe prolonged deficiency causes rickets in children and osteomalacia in adults.
What moves it
Season above everything: in northern Europe the skin makes essentially none between October and March. Then sunscreen and clothing, skin tone, age, body weight and pregnancy. Supplements take weeks to move the number, so retesting sooner than about three months shows little.
What usually happens next
A deficient result usually leads to replacement, then a recheck after about three months if the cause is unclear or absorption is in doubt. If calcium is abnormal too, the picture gets investigated further — parathyroid hormone, kidney function — because vitamin D, calcium and PTH only make sense read together.
Common questions
Should everyone have their vitamin D tested?
Most guidelines say no. Where deficiency is very likely — winter at a northern latitude, little sun exposure — many clinicians simply supplement, because the test costs more than the vitamin. Testing is for people with symptoms, malabsorption, bone disease or an abnormal calcium.
ng/mL or nmol/L — how do I compare them?
Multiply ng/mL by 2.5 to get nmol/L. So 20 ng/mL is 50 nmol/L and 30 ng/mL is 75 nmol/L. Reports from the United States usually print ng/mL and most of Europe uses nmol/L, which is a frequent source of alarm about a perfectly ordinary result.