Vitamin D blood levels: how much is good, how much is too little

Vitamin D status is measured as serum 25-hydroxy-D3 (25(OH)D). About 70–80% of people in Estonia are deficient in winter. But the “normal range” the lab prints doesn’t equal “optimal”.

Reference range vs optimal

• < 25 nmol/L (10 ng/mL) – severe deficiency
• 25–50 nmol/L (10–20 ng/mL) – deficiency
• 50–75 nmol/L (20–30 ng/mL) – borderline – labs say “normal”, it is NOT optimal
• 75–125 nmol/L (30–50 ng/mL) – optimal (Endocrine Society, Holick 2007)
• > 250 nmol/L (100 ng/mL) – toxicity possible

Why deficiency is the norm in Estonia

Skin only makes vitamin D under UVB (290–315 nm). In Estonia (~59° N) UVB is only sufficient from May to September at midday. October to March, skin produces essentially none.

How a D3 dose is determined