Vitamin D Deficiency: How to Interpret Your 25-OH Results

Vitamin D deficiency is one of the most prevalent nutrient deficiencies worldwide, affecting an estimated 1 billion people across all age groups and ethnicities. Despite its importance in bone health, immune function, and chronic disease prevention, it remains massively under-diagnosed. The 25-hydroxy Vitamin D test (25-OH D) is the definitive way to assess your Vitamin D status.

Understanding the Test

25-hydroxyvitamin D [25(OH)D] is the main circulating form of Vitamin D and the best indicator of total body stores. It is produced in the liver from Vitamin D obtained through sunlight exposure and diet. The result is reported in ng/mL (conventional units) or nmol/L (SI units). The conversion factor is: 1 ng/mL = 2.496 nmol/L.

Interpreting Your Result

The Endocrine Society and most clinical guidelines use these cut-points:

• < 10 ng/mL (< 25 nmol/L): Severely deficient — high risk of rickets/osteomalacia
• 10–20 ng/mL (25–50 nmol/L): Deficient — bone disease risk, immune impairment
• 20–30 ng/mL (50–75 nmol/L): Insufficient — may benefit from supplementation
• 30–100 ng/mL (75–250 nmol/L): Sufficient — optimal range for most adults
• > 100 ng/mL (> 250 nmol/L): Potentially toxic — calcium should be checked
• > 150 ng/mL (> 375 nmol/L): Toxic — vitamin D toxicity, hypercalcaemia risk

Who Is Most at Risk?

Certain groups are at particularly high risk of deficiency:

• People with darker skin (more melanin reduces UV-driven synthesis)
• Those living at high latitudes with limited sun exposure
• Elderly individuals (reduced skin synthesis capacity)
• People who cover their skin for cultural or religious reasons
• Those with obesity (Vitamin D is fat-soluble and sequestered in adipose tissue)
• Patients with malabsorption (Crohn's, coeliac disease, gastric bypass)

Related Tests

Vitamin D does not work in isolation. When investigating deficiency or suspected bone disease, consider these related tests:

• PTH (Parathyroid Hormone): rises in Vitamin D deficiency to maintain calcium
• Calcium (serum): may be low in severe deficiency; high in toxicity
• Phosphorus: low in rickets and osteomalacia
• Alkaline Phosphatase (ALP): elevated in active bone disease