Determination and Assessment of Vitamin D Status: Basics and References Values
The vitamin D status is determined by measuring 25-hydroxyvitamin D (25(OH)D) in the blood serum. This marker reflects both endogenously produced and dietary or supplemental sources of vitamin D. 25(OH)D has a half-life of about 15 days in serum and is measured in nanomoles per liter (nmol/L) or nanograms per milliliter (ng/mL), where 1 nmol/L equals 0.4 ng/mL, and 1 ng/mL equals 2.5 nmol/L. The determination of vitamin D status via 25(OH)D may vary due to different tests used by various laboratories. This can result in discrepancies where results may appear too low or too high depending on the method and laboratory. To address these challenges, an international standardization program was developed to harmonize laboratory measurements in order to promote public health.
In contrast, the circulating form 1,25-dihydroxyvitamin D (1,25(OH)2D) is not a reliable indicator of vitamin D status, as it has a short half-life, and its values are tightly regulated by hormones such as thyroid hormone, calcium, and phosphate. 1,25(OH)2D levels generally decrease only in cases of severe vitamin D deficiency. Although 25(OH)D serves as a biomarker for vitamin D exposure, it is unclear to what extent these levels reflect overall health status. An expert panel of the National Academies of Sciences, Engineering, and Medicine (NASEM) concluded that serum concentrations below 30 nmol/L (12 ng/mL) are associated with a risk of vitamin D deficiency, while levels between 30 and 50 nmol/L (12–20 ng/mL) may be considered insufficient. For most people, levels of 50 nmol/L (20 ng/mL) or more are considered adequate. Additionally, concentrations above 125 nmol/L (50 ng/mL) may indicate potential side effects. However, the Endocrine Society has not set specific thresholds for 25(OH)D regarding sufficiency, insufficiency, or deficiency and does not recommend routine testing for healthy individuals.
Various reference values are used to assess 25(OH)D levels. The Robert Koch Institute follows the classification of the U.S. Institute of Medicine (IOM), which focuses on bone health:
Supply Assessment | 25-Hydroxyvitamin D in Blood (nmol/L or ng/mL) | Clinical Significance/Effect |
|---|---|---|
Severe Deficiency | <25 nmol/l (<10 ng/ml) | Risk of bone mineralization disorder / rickets / osteomalacia |
Insufficient Supply | 25-49 nmol/l (10-19 ng/ml) | Risk of increased bone turnover and/or rise in parathyroid hormone |
Vitamin D Deficiency | <50 nmol/l (< 20 ng/ml) | Includes insufficient supply and severe deficiency |
Adequate Vitamin D Supply (minimum concentration) | 50 nmol/l (20 ng/ml) | Low risk of bone loss and parathyroid hormone increase; neutral effect on falls and fractures |
Target Value for Reducing Falls and Fractures | 75 nmol/l (30 ng/ml) | Suppression of parathyroid hormone increase and bone resorption, reduction of falls and fractures |
Throughout this article, it becomes clear that vitamin D status is determined by measuring 25-hydroxyvitamin D (25(OH)D) in blood serum. Levels below 25 nmol/L indicate a severe deficiency, while levels above 50 nmol/L are considered sufficient. A target level of 75 nmol/L is recommended for reducing the risk of falls and fractures. Various laboratory methods may yield differing results, making standardization necessary. Routine testing in healthy individuals is not recommended; the focus is on preventing bone-related issues such as rickets and osteomalacia.
Sources:
National Institute of Health. (2024). Office of Dietary Supplements—Vitamin D. Vitamin D Fact Sheet for Health Professionals. Source
Robert Koch Institut. (2026). Vitamin D. Source
Schweizerische Eidgenossenschaft Bundesamt für Lebensmittelsicherheit und Veterinärwesen BLV Lebensmittel und Ernährung. (o. J.). Fachinformation zu Vitamin D. Abgerufen 20. September 2024, von Source