There's at least two different lab tests for vitamin D, one that measures the level in its inactive form, and one that measures the level in its active form. The inactive level is what you'll get for a routine test. The other test takes a long time to process and is more expensive, and in my experience, health insurance really doesn't like having to pay for it. The active level is ultimately what's important because it's what's interacting with your bodily functions. The tricky bit is that the relationship between the two levels is fairly inconsistent from individual to individual, and variably determined by poorly understood mechanisms. A decent subset of the population has under-diagnosed inflammatory diseases, and for whatever reason inflammatory disease tends to increase the ratio of active to inactive vitamin D. Outside of rheumatology, this dynamic isn't really known about by most doctors. There's therefore a non-zero risk of vitamin D poisoning from supplementing based purely on low test results from a primary care provider. Regulating your inactive level to the "normal" range may unknowingly increase your active level to the point that it causes problems like hypercalcemia and eventual bone damage.