Ethnicity may make a difference in vitamin D levels; African-Americans typically have lower levels, but it's not known if that's deficient or OK.
And then there's the fact that the blood test can be expensive, running $50 to $220. Since vitamin D supplements are cheap, some doctors say it's easier just to tell people to take supplements rather than mess around with a blood test.
In 2010, the Institute of Medicine issued a report saying that vitamin D is vital for bone health, but other than that there isn't evidence that low levels of vitamin D cause disease. It recommended that adults up to age 70 get 600 milligrams of D a day, with 800 milligrams for people over 70.
People can make vitamin D when exposed to sunshine. But many people in North America never get enough sun to do that
Always talk with your healthcare provider before taking any Vitamin supplements.