Vitamin D Deficiency: Signs, Testing & How Much to Take

Quick Answer

Vitamin D deficiency (25(OH)D below 20 ng/mL) causes fatigue, bone pain, low mood, and weakened immunity. Test your level first, then supplement with 2000–4000 IU D3 daily to reach 40–60 ng/mL. Always take with vitamin K2 (100–200 mcg MK-7) and a fatty meal.

Key Takeaways

• 41% of US adults are deficient; 82% of dark-skinned people and those in northern climates
• Standard blood test: 25-hydroxyvitamin D (25(OH)D) — not 1,25-dihydroxyvitamin D
• Target: 40–60 ng/mL (100–150 nmol/L) — most functional medicine physicians recommend the upper end
• D3 (cholecalciferol) is 87% more potent than D2 (ergocalciferol) at raising serum levels
• Takes 8–12 weeks of consistent supplementation to meaningfully shift levels

Symptoms of Deficiency

Commonly attributed to D deficiency (with some evidence):

• Persistent fatigue and low energy
• Bone and muscle pain (osteomalacia)
• Frequent infections and slow recovery
• Low mood, seasonal depression
• Hair loss and slow wound healing

Note: These symptoms overlap with many conditions. Testing is the only way to confirm deficiency.

How to Test

Ask your GP or use a home finger-prick test (Thriva, Medichecks). Request specifically: 25-hydroxyvitamin D (25(OH)D) — not the active 1,25 form, which doesn't reflect stores.

Supplementation Protocol

Correcting deficiency (below 30 ng/mL):

• 4000 IU D3 daily for 12 weeks, then retest
• Take with 100–200 mcg vitamin K2 (MK-7) and a fat-containing meal

Maintenance (30–40 ng/mL):

• 2000 IU D3 daily

Already sufficient (40+ ng/mL):

• 1000 IU D3 or sun exposure may suffice

Why K2 Is Non-Negotiable

High-dose D3 without K2 increases calcium absorption but cannot direct that calcium to bones (osteocalcin requires K2-activated carboxylation). This risks soft-tissue calcification. Always take D3 with K2 MK-7.

Sunlight vs Supplements

In summer, fair skin produces ~10 000 IU in 15 minutes of midday full-body exposure. However: sunscreen blocks ~95% of synthesis, glass blocks UVB completely, and above 50° latitude there is insufficient UVB October–March. Supplementation is the reliable solution year-round for most people.