Vitamin D, 25-Hydroxy

You can get vitamin D from three sources: sunlight (your body makes vitamin D when bare skin is exposed to sun), certain foods (egg yolks, liver, fatty fish such as salmon, and foods that are fortified with vitamin D such as cereals and milk), and supplements in pill or liquid form.

Before your body can use vitamin D, your liver must change it into another form called 25 hydroxyvitamin D, also known as 25(OH)D. Most vitamin D blood tests measure the level of 25(OH)D in your blood, because it is the most accurate way to measure how much vitamin D is in your body. The 25(OH)D level is generally a good reflection of overall vitamin D activity, except in people with kidney disease. Another type of vitamin D test measures "active vitamin D," which is also called calcitriol or 1,25-dihydroxyvitamin D. This test isn't usually used to check whether you are getting enough vitamin D, but it may be used to monitor people with certain kidney problems or abnormal blood calcium levels.

Your provider may order a vitamin D test if you have signs of a condition that may be caused by very low levels of vitamin D, such as osteomalacia (a softening of the bones in adults), low bone density, osteopenia, osteoporosis, or rickets in children. You may also need this test if you have a higher risk for vitamin D deficiency due to limited sun exposure, certain medications, weight-loss surgery, malabsorption disorders such as Crohn's disease or celiac disease, or kidney/liver disease.

Test results are reported as "total vitamin D" or as separate amounts of vitamin D2 and vitamin D3. The total vitamin D is the important number to check.

In general, low levels of vitamin D in your blood mean one or more of these things: You don't get enough vitamin D from sunlight or your diet, your digestive tract is having trouble absorbing vitamin D, you are having trouble changing vitamin D into a form your body can use (this could mean you have liver or kidney disease), or you are taking certain medicines that affect vitamin D.

As a general guide, levels below about 25 ng/mL are linked to a higher risk of thinning bones and fractures, and levels below 10 ng/mL can be severe enough to cause soft, weak bones (rickets in children, osteomalacia in adults). Very high levels — usually above 80 to 150 ng/mL — point to too much vitamin D, almost always from high-dose supplements.

Vitamin D levels are often described as: deficient (vitamin D levels are very low and can affect your bones and overall health), insufficient (low levels that may weaken your bones over time even if you have no symptoms), sufficient/optimal (good levels for healthy bones and general health), or toxic/possible toxicity (very high levels that might cause health problems).

If your test shows that you have low vitamin D levels, your provider may recommend that you take vitamin D supplements and/or eat more foods that have vitamin D. If your levels are very high, your provider will tell you to stop taking vitamin D supplements. Too much vitamin D in your blood can damage your organs and blood vessels, including by causing high blood calcium levels.