Vitamin D (25-OH) — Normal Range & Interpretation

Clinical Context

Serum 25-hydroxyvitamin D measures the body's total vitamin D stores from dietary intake, supplementation, and cutaneous synthesis. It serves as the best indicator of vitamin D status because of its long half-life and stable circulating levels. Clinicians use it to evaluate bone health, calcium and phosphorus metabolism, and risk for osteomalacia, osteoporosis, and rickets.

Deficiency arises from inadequate sun exposure, darker skin pigmentation, obesity, malabsorption syndromes such as celiac and Crohn disease, chronic kidney disease, liver disease, and medications including phenytoin, rifampin, and glucocorticoids. Older adults and exclusively breastfed infants without supplementation carry elevated risk. Elevated levels almost always reflect excessive supplementation rather than dietary intake or sun exposure, and toxicity produces hypercalcemia, nephrocalcinosis, and soft-tissue calcification.

FNPs see vitamin D on the boards most often in questions on deficiency thresholds, with levels under 20 ng/mL indicating deficiency and 20 to 29 ng/mL reflecting insufficiency. Expect questions linking low vitamin D to secondary hyperparathyroidism, bone pain, proximal muscle weakness, and falls in older adults. Know that cholecalciferol (D3) raises levels more effectively than ergocalciferol (D2), standard replacement uses 50,000 IU weekly for 8 weeks in deficient patients, and screening targets high-risk groups rather than the general population.

Related Labs

• HbA1c — %
• ESR — mm/hr
• CRP — mg/dL
• hs-CRP — mg/L
• Uric Acid — mg/dL

Sources

• MedlinePlus