Vitamins, minerals and amino acids
Vitamine D (25-OH)
The storage form of vitamin D; shows whether there is a deficiency, but in healthy people the result usually does not change the advice.
Everything about this testWhat it measures
Vitamin D is mainly made in the skin under the influence of sunlight and comes for a small part from food (oily fish, margarine, eggs). The liver converts it into 25-hydroxyvitamin D, the storage form that this analysis measures; the kidneys turn that into the active form. Vitamin D is needed for the absorption of calcium and phosphate and thus for strong bones and muscles. The value is lowest at the end of winter and highest in late summer.
- Vitamin D deficiency, which with a long-term severe deficiency can cause softening of the bones (osteomalacia), muscle weakness and, in children, rickets
- Too much vitamin D from high-dose supplements (rare)
- An explanation for an abnormal calcium or parathyroid hormone (PTH)
High
A raised 25-OH vitamin D is caused almost only by high-dose supplements. Only at very high values, far above the target value, can the calcium in the blood rise, with nausea, thirst, frequent urination, confusion and, in the long run, kidney damage. Sunlight alone does not cause too high a value, because the skin limits production itself.
Low
A low 25-OH vitamin D is very common at the end of winter, in people who rarely go outside or who cover their skin, with dark skin, at older age and with overweight. According to the Health Council of the Netherlands, in adults up to 70 years a value below 30 nmol/L counts as a deficiency, and above 70 years a value below 50 nmol/L. A slightly low value usually causes no symptoms; a long-term severe deficiency can lead to bone pain, muscle weakness, falling more often and fractures. Symptoms such as tiredness are often attributed to vitamin D, but the evidence that a supplement improves those symptoms is weak. A deficiency due to poor absorption occurs with bowel diseases such as celiac disease and Crohn's disease, after bariatric surgery and with some antiepileptic drugs.
Preparation
- Do you take biotin (vitamin B8, often in hair and nail supplements)? Stop at least 2 days before the sample is taken, or 4 days for a high dose.
If your result is abnormal
With a deficiency, the GP usually advises a vitamin D supplement and a follow-up measurement is usually not needed; with a severe deficiency or a condition causing poor absorption, a check after a few months sometimes follows, along with tests of calcium, phosphate and PTH. With a strongly raised value, supplement use is checked and calcium is measured.
Reliability
Measuring 25-OH vitamin D is the recognized measure of vitamin D status, but the result differs between laboratories and measurement methods and varies with the season. According to the Dutch GP association's position statement (NHG) on diagnosing vitamin D deficiency, the result does not change the advice in healthy people without symptoms, and for the risk groups of the Health Council of the Netherlands a supplement advice applies regardless of the result. The result says the most with symptoms that fit a severe deficiency, with conditions causing poor absorption and with an abnormal calcium.
Limitations
- Season: the value is lowest at the end of winter and highest in late summer; a measurement in March and one in September are not comparable.
- Laboratories and measurement methods differ; shortly after a high dose of a supplement, the value is temporarily higher than the true stores.
- With a raised or lowered protein level (pregnancy, liver disease, kidney disease), the total value says less about the available vitamin D.
- Antiepileptic drugs, some HIV medicines and fat-binding agents lower the value; biotin supplements can disturb the measurement.
How common is this
At the end of winter, about 7 to 26 in 100 Dutch adults have a vitamin D deficiency, depending on the cut-off used; in summer this drops to 0 to 5 in 100. Men have a deficiency more often than women, and people with dark skin, older people in care homes and people who rarely go outside much more often. A severe deficiency with bone symptoms is rare.
Who is this useful for
Useful with symptoms that fit a severe deficiency (bone pain, muscle weakness, frequent falls), with osteoporosis or a fracture, with conditions causing poor absorption (celiac disease, Crohn's disease, after bariatric surgery), with an abnormal calcium and for monitoring treatment with high doses. For the risk groups of the Health Council of the Netherlands (children up to 4 years, pregnant women, women over 50, everyone over 70, people with dark skin or who rarely go outside), a supplement advice of 10 to 20 micrograms per day applies, regardless of the result. In healthy adults without symptoms or risk factors the result says little, partly because a slightly low value at the end of winter is very common.
Downsides and risks
- With a slightly low value in winter, a normal seasonal effect is easily read as a 'deficiency'; in healthy people without symptoms the result does not change the advice (whether or not to take a supplement).
- Taking high-dose supplements on your own initiative after a low result can lead to too much calcium.
Sources
- NHG-Standpunt Diagnostiek van vitamine-D-deficiëntie (2012)
- NVKC, allesovertesten.nl: Vitamine D
- Gezondheidsraad: Evaluatie van de voedingsnormen voor vitamine D (2012)
- RIVM: Maximaal een kwart van de Nederlandse bevolking heeft vitamine D-tekort in de winter (2025)
- Thuisarts.nl: Vitamine D
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results