25-hydroxyvitamin D (25(OH)D) is the primary indicator of vitamin D levels in the body. Vitamin D is essential for normal calcium absorption and the maintenance of healthy bones, muscles, and the immune system. This test helps assess the body's vitamin D status and identify any vitamin D deficiency or excess. Vitamin D testing is also used to monitor levels during treatment.
Vitamin D helps the body absorb calcium, magnesium, and phosphorus, which are critical for bone strength and overall health. Its deficiency can cause muscle and bone pain and brittle bones. The unique feature of vitamin D is that it can be produced in the human body when exposed to sunlight. It can also be obtained from food or special supplements.
There are two main forms of vitamin D:
- D2 (ergocalciferol), which comes mainly from food and supplements;
- D3 (cholecalciferol), which is formed in the skin when exposed to sunlight.
After entering the body, vitamin D is converted into various metabolites, among which 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D are of greatest clinical importance.
25-hydroxyvitamin D is the main form used to assess the body's vitamin D status. This indicator is the most informative biochemical marker of its status. World guidelines, in particular the Association of Endocrinologists, recommend the determination of 25(OH)D during a blood test for vitamin D.
1,25-dihydroxyvitamin D is an active hormonal form of vitamin D, which is formed mainly in the kidneys. It is not usually used to diagnose vitamin D deficiency, but the determination can be useful for clarifying the causes of calcium metabolism disorders, in particular in kidney diseases or rare hereditary conditions.
In case of detected deficiency, the result of a blood test for vitamin D should be interpreted together with the determination of the level of calcium, phosphorus, magnesium, alkaline phosphatase, parathyroid hormone and creatinine.
Vitamin D normal level
The level of 25-hydroxyvitamin D can vary depending on age, lifestyle, dietary habits, and time spent in the sun at different times of the year. A reduced level may indicate a deficiency or deficiency of vitamin D, and a significant increase may indicate its excess and the risk of toxic effects.
Criteria for vitamin D adequacy*:
- < 20 ng/ml (< 50 nmol/l) – deficiency;
- ≥ 20 ng/ml (≥ 50 nmol/l) and < 30 ng/ml (< 75 nmol/l) – insufficiency;
- 30-50 ng/ml (75–125 nmol/l) – sufficient level;
- > 50-60 ng/ml (> 125–150 nmol/l) – safe, but not target level;
- > 60-100 ng/ml (> 150–250 nmol/l) – zone of uncertainty with potential benefits or risks;
- > 100 ng/ml (> 250 nmol/l) – excess/zone of toxicity.
When interpreting the results of a vitamin D test, it is important to pay attention to the units of measurement in the laboratory (ng/ml or nmol/l) in order to correctly assess the level of the indicator. If you need to take vitamin D tests again, it is advisable to perform them in the same laboratory for correct comparison of results.
Why is the level of vitamin D elevated and what does it affect?
An elevated level of 25(OH)D is most often associated with excessive or uncontrolled intake of drugs or supplements with vitamin D. Much less often, the cause may be individual metabolic disorders.
Excess can lead to hypercalcemia (increased calcium levels in the blood) and cause:
- kidney damage (stone formation, dysfunction);
- calcium deposits in blood vessels and soft tissues;
- heart rhythm disorders;
- symptoms of intoxication – nausea, weakness, thirst.
Causes and signs of vitamin D deficiency
The deficiency develops gradually and remains unnoticed for a long time. The most common causes: insufficient intake with food, limited exposure to the sun (especially in autumn and winter), impaired absorption in the intestines.
The main groups at increased risk include the elderly, pregnant and breastfeeding women, patients with chronic diseases of the gastrointestinal tract, liver or kidneys, as well as overweight individuals.
Insufficient levels of vitamin D can manifest themselves in the following symptoms:
- general weakness, fatigue, decreased performance;
- pain in muscles, joints and bones;
- decreased bone density, increased bone fragility;
- in children – rickets, in adults – osteomalacia (softening of bone tissue due to impaired mineralization) or progression of osteoporosis (decreased bone density and susceptibility to fractures).
*Diagnosis, prevention and treatment of vitamin D deficiency in adults: Consensus of Ukrainian experts, 2023
