The reference range for vitamin D is 25 to 80 ng/mL according to Mayo Medical Laboratories. This range represents optimal vitamin D levels for adults. Levels above 80 ng/mL are considered toxic. Levels between 10 and 25 ng/mL are mild to moderately deficient. Levels below 10 ng/mL are severely deficient.
Vitamin D is important for bone health, for the absorption of calcium and phosphorus and for muscle and cardiovascular function, explains the Vitamin D Council. Vitamin D also has anti-cancer properties, supports the immune and respiratory systems and contributes to brain health.
A lack of exposure to sunlight, a history of kidney or liver disease, poor food absorption, a lack of vitamin D in the diet and the use of certain medications can contribute to low vitamin D levels, notes MedlinePlus.