Vitamin D Test: What Your 25-OH Result Means

Vitamin D is a fat-soluble vitamin that acts like a hormone in the body. The 25-hydroxyvitamin D (25-OH vitamin D) blood test is the standard way to measure your vitamin D status because it reflects both the vitamin D you make from sun exposure and what you get from food and supplements.

Unlike some nutrients, vitamin D can be produced when skin is exposed to UVB sunlight. However, sunscreen, darker skin, higher body weight, aging, and living at northern latitudes all reduce how much your body makes.

Low vitamin D is one of the most common nutrient deficiencies. It is linked to fatigue, low mood, weakened immune response, and bone loss over time. In women, low vitamin D can accelerate postmenopausal bone loss.

Very high vitamin D, usually from excessive supplementation, can cause elevated blood calcium and kidney stress. Because vitamin D is fat-soluble, it can accumulate, so more is not always better.

The main natural source is sunlight on bare skin. Dietary sources include fatty fish (salmon, mackerel, sardines), egg yolks, fortified milk and plant milks, and fortified cereals. Some people also use vitamin D3 supplements, especially in winter or when sun exposure is limited.

Season and latitude, skin tone, body weight, age, sunscreen use, digestive health, liver and kidney function, and certain medications can all affect vitamin D levels.