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Vitamin D functions more like a hormone than a vitamin — your skin synthesizes it from UVB sunlight, and it regulates immune function, mood, bone density, calcium absorption, insulin sensitivity, and inflammation throughout the body. Deficiency is extremely common: an estimated 1 billion people globally have insufficient levels, largely because modern indoor lifestyles limit sun exposure. A blood level below 20 ng/mL is clinically deficient; most functional medicine guidelines suggest 40–60 ng/mL for optimal health.
Also known as: 25 Hydroxy Vitamin D Total, 25-hydroxycholecalciferol, 25-Hydroxyvitamin D, 25-hydroxyvitamin D 3, 25-Hydroxyvitamin D Total, 25-Hydroxyvitamin D, Total, 25-Hydroxyvitamin D3, 25-OH Vit D
High vitamin D (above 100 ng/mL) can cause hypercalcemia, kidney stones, and vascular calcification. Low vitamin D impairs immunity, mood, bone density, and increases cancer and cardiovascular risk.
Standard labs consider above 30 ng/mL sufficient. Functional practitioners target 50-80 ng/mL. Most adults need 2000-5000 IU daily to maintain optimal levels.