Vitamin D Testing

A normal vitamin D blood test result does not confirm adequate levels of the active free fraction. Conventional labs flag vitamin D levels above 100 ng/mL as potentially toxic, a threshold described as potentially arbitrary given therapeutic evidence. The biologically active free fraction of vitamin D turns over in just 24 hours. Standard vitamin D blood tests measure only the inactive form, not the metabolically active free fraction. Bound vitamin D has a 2–3 week half-life that allows it to accumulate and create the appearance of sufficiency even when the active fraction is depleted. A patient can have normal blood vitamin D levels and still be functionally vitamin D deficient at the cellular level. Serum testing reveals circulating vitamin D levels but nothing about what is happening at the receptor level in tissues and organs. Standard vitamin D blood tests measure 25-hydroxyvitamin D in serum and cannot reveal receptor-level dysfunction.