Vitamin D is a nutrient found in food, supplements, and ultraviolet sunlight. Along with its cousin, calcium, it helps bones grow and remain strong.
Fish oil, swordfish, tuna and fortified dairy products are all foods high in vitamin D. 
Vitamin D deficiency--which happens when you do not have enough of the nutrient in your body--can cause soft bones and osteoporosis. It can also weaken your immune system, increasing the risk of many illnesses and chronic conditions.
If you live north of a line connecting San Francisco and Philadelphia, and are not in the sun for at least 15 minutes a day, your vitamin D level is probably lower than normal. People who have dark skin, are older, or are overweight are also at a greater risk of having low levels of this vitamin.
One billion people have vitamin D deficiency worldwide; in the United States, it’s one in every four people.
Does this mean that tests that measure vitamin D are widely accessible?
Not always, but until recently, there’s been a good reason for that: experts believe that mass testing only leads to excessive, and at times unnecessary, treatments. The problem, others say, is that the people who really need the test might not be able to access it.
A blood test is the only way of knowing whether a person suffers from vitamin D deficiency. Values between 20 and 50 nanograms per milliliter are considered normal. A value of less than 12 ng/mL is indicative of deficiency.
On average, vitamin D tests cost $50, but in some specific cases, they can cost up to $180.
These tests are usually covered by insurers, but not always at the time the consumer or doctor needs them to be.
For example, there are insurers who won’t cover this test if it’s ordered during a patient’s annual check-up, which seems rather illogical. Consumers have reported receiving bills of up to $600 for this test, just because they had it done during the only time of year they see their doctor...
Even Medicare doesn’t always cover it as a routine test, even though vitamin D can be an indicator for some age-related conditions.
Other insurers require a doctor’s explanation of the clinical reasons for this test.
Depending on what health plan you have, you might have to pay for the test out-of-pocket and then request at least a partial reimbursement.
Because there’s no set rule, here’s what we recommend: when you go in for your annual check-up, ask your primary care physician if you should get a vitamin D test, and then ask him or her to check with your insurer first to see if it’s covered and how much it costs. That way, you'll avoid any surprises.
Along with your doctor, you can figure out the right time to have the test at little or no cost.
Sources: NIH Office of Dietary Supplements, Harvard T.H. Chan School of Public Health, CMS
