Laboratory test · Men’s health
Vitamin D
- Sample: venous blood
- Preparation: fasting not essential
Test price
- Sample
- venous blood
- Preparation
- fasting not essential
Paymentcash, cards and instalments
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The vitamin D test measures 25-hydroxyvitamin D, or 25(OH)D, the form circulating in blood that best reflects how much vitamin D the body has. It is used for suspected vitamin D deficiency, bone disease and conditions affecting absorption, and to monitor replacement.
Who the test is for
- people with bone pain and muscle weakness that may suggest osteomalacia, and children with suspected rickets;
- people with bone disease, for example before certain osteoporosis medicines administered by infusion or injection;
- people with abnormal calcium, phosphorus, PTH or alkaline phosphatase;
- people with conditions affecting absorption, such as coeliac disease and inflammatory bowel disease, and after bariatric surgery;
- people with chronic kidney or liver disease and those taking certain medicines long term, such as some antiepileptics;
- people taking replacement when a doctor needs to check its effect, for example with poor absorption or persistent symptoms.
What vitamin D is and what the test measures
Most vitamin D is produced in the skin through sunlight, while smaller amounts come from foods such as oily fish, egg yolks and fortified products, and from supplements. The liver converts vitamin D to 25(OH)D, and the kidneys to the active form, which together with parathyroid hormone (PTH), regulates blood calcium and phosphorus. Without enough vitamin D, bones cannot mineralise properly: children may develop rickets and adults osteomalacia, or softening of the bones.
Assessment uses 25(OH)D because it stays in blood for several weeks and reflects intake and sun exposure over a longer period. The active form is not a good indicator of stores: it is short-lived and tightly regulated, so may be normal even in vitamin D deficiency.
Routine vitamin D testing is not recommended for healthy people without symptoms or specific risk factors. The American Endocrine Society’s 2024 guideline advises against routine testing even in the groups it covered, including pregnant women and people over 75. The test is useful when results affect treatment.
Save with a panel
Test Vitamin D is also included in these panels.
Preparation and sample
Vitamin D is measured in venous blood, and fasting is not required. If you take vitamin D, do not stop before testing; instead tell your doctor the dose and form, for example drops, tablets or ampoules. When checking replacement effectiveness, follow-up is usually performed no earlier than approximately three months after starting or changing the dose, once levels stabilise.
Vitamin D varies through the year: it is usually lowest in late winter and early spring and highest in late summer. This should be considered when comparing two results.
How results are interpreted
Vitamin D is expressed in nmol/L or ng/mL (1 ng/mL equals 2,5 nmol/L). Cut-offs for deficiency and adequacy differ between guidelines, so they should be read as a framework rather than rigid boundaries. According to the American Institute of Medicine (IOM), whose limits are also quoted by the US National Institutes of Health (NIH):
| 25(OH)D | Most common meaning |
|---|---|
| below 30 nmol/L (12 ng/mL) | deficiency, with a risk to bone health; British guidelines use a deficiency cut-off of 25 nmol/L |
| 30 to 50 nmol/L (12 to 20 ng/mL) | insufficient for some people |
| 50 nmol/L (20 ng/mL) and above | adequate for bone health in most people |
| above 125 nmol/L (50 ng/mL) | no proven additional benefit; very high levels may have adverse effects |
Many laboratories, including in Croatia, list 75 nmol/L (30 ng/mL) or above as desirable and flag lower values as insufficient or deficient. The Endocrine Society’s 2024 guideline does not set a target value. The range on your report may therefore differ from the table, and the range printed on your report applies to your result. A doctor interprets it alongside symptoms, calcium, PTH and other tests.
If vitamin D is low, a doctor sets the dose and duration of replacement. Do not take high doses, for example from ampoules, on your own initiative. Very high levels are almost always caused by excessive supplements and may raise blood calcium, causing nausea, thirst and frequent urination.
How to arrange testing
You can have blood taken without an appointment or referral at one of our branches across Croatia (listed on the Contact page). Prices for all tests and panels are available in the service price list. Vitamin D is also measured as part of the Energy Balance Panel, alongside vitamin B12, folic acid, calcium, magnesium, iron and electrolytes. Free consultations are available with your results.
Frequently asked questions
Should I stop taking vitamin D before testing?
No. The test shows your level on the dose you take. Do not stop without medical agreement, and tell your doctor how much vitamin D you take.
Does the test measure the active form of vitamin D?
No. It measures 25(OH)D, which best reflects vitamin D stores. The active form, 1,25-dihydroxyvitamin D, is measured only in specific situations, such as certain kidney diseases or calcium disorders, on a specialist’s advice.
Why is vitamin D not measured in everyone?
In healthy people, results rarely change recommendations, and cut-offs differ between guidelines. Testing is therefore used when symptoms, disease or medicines affect vitamin D, or when a doctor needs to check replacement effectiveness.
When is the best time to measure vitamin D?
Vitamin D can be measured at any time of day or year. To compare two results, it is useful to test at the same time of year because levels are lower in late winter than late summer.