What does the 25-Hydroxyvitamin D, Total test measure?

Why is 25-Hydroxyvitamin D, Total measured?

25(OH)D is the standard assay for evaluating vitamin D status. Deficiency impairs intestinal calcium absorption and can lead to secondary hyperparathyroidism, bone loss, osteomalacia in adults, and rickets in children. Insufficiency has been linked to increased fracture risk, muscle weakness, and potentially to immune, cardiovascular, and metabolic outcomes, though causality for extra-skeletal conditions remains contested. The test guides repletion therapy and is essential in the investigation of metabolic bone disease, unexplained hypocalcaemia, and chronic kidney disease-mineral bone disorder.

Reference ranges

Standard reference

Deficiency <50 nmol/L (<20 ng/mL); insufficiency 50–74 nmol/L (20–29 ng/mL); sufficiency ≥75 nmol/L (≥30 ng/mL). Laboratory-specific thresholds may vary.

Functional medicine optimal

The Endocrine Society defines sufficiency as ≥75 nmol/L (≥30 ng/mL), but other bodies recommend ≥50 nmol/L (≥20 ng/mL) for skeletal health. Functional-medicine practitioners sometimes target 100–150 nmol/L (40–60 ng/mL), a range not universally endorsed and requiring individualised risk-benefit assessment.

Units

nmol/L, ng/mL

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.