What Is Vitamin D in a Blood Test?

Vitamin D testing measures 25-hydroxyvitamin D (25-OH D) — the storage form and best indicator of vitamin D status.

Reference Ranges

• Severe deficiency: below 12 ng/mL
• Deficiency: 12–20 ng/mL
• Insufficiency: 20–30 ng/mL
• Sufficient: 30–50 ng/mL
• Toxic: above 100 ng/mL

Why Vitamin D Matters

• Bone health — calcium absorption
• Immune function
• Muscle function
• Mood
• Cardiovascular health

Deficiency Causes

• Limited sun exposure
• Dark skin — melanin reduces synthesis
• Obesity
• Malabsorption — celiac, Crohn’s, gastric bypass
• Kidney/liver disease
• Age
• Medications — anticonvulsants

Supplementation

• RDA: 600–800 IU/day
• Deficiency treatment: 50,000 IU/week for 8 weeks
• Recheck level 8–12 weeks after starting