The test measures 25-hydroxyvitamin D, the form the liver makes from vitamin D produced in skin under sunlight or taken in with food and supplements. It circulates long enough to reflect stores, which makes it the accepted measure of overall vitamin D supply. That supply underlies calcium and phosphate handling, bone mineralization and parathyroid regulation.
No readings for this marker yet.
Low values most often come down to little sun exposure, winter at northern latitudes, low dietary intake, or poor absorption after gut disease or bariatric surgery; body fat holds vitamin D away from circulation, and some medicines speed its breakdown. Prolonged deficiency is what pushes parathyroid hormone up and weakens bone mineralization. High values almost always trace back to supplements rather than to sun or food. Either direction is a reason to recheck and to bring the result to a doctor.
The result swings with season — a sample drawn at the end of winter runs lower than the same person's late-summer one — and a recent large supplement dose or an acute illness shifts it as well; assays differ between laboratories, so a trend is best followed at one lab. It sits on the same vitamin panel as vitamin B12 and folate but is not interpreted with them: its clinical company is calcium, phosphate, parathyroid hormone and alkaline phosphatase.
Reference ranges are general adult values and depend on lab, assay, age and sex. For orientation, not diagnosis — discuss results with your doctor.