Some drugs deplete Vit D from the body.
Antiseizure drugs
H2 antagonists (Zantac/Axid/Tagamet are listed)
Predinsone and other steroids
Rifampin (antibiotic)
Orlistat (OTC Alli diet pill)
INH (TB drug)
Barbiturates (not used commonly today)
there may be others not studied enough yet as well

I suggest people who are using D3 to use a gelcap/liquigel formula and not a dry cap or tablet. I believe the gelcap/liquigel type is more completely absorbed.

Confusing testing is occurring with D3 and Quest labs.
So many patients may be getting erroneous test results:
http://community.wegohealth.com/prof...st-diagnostics

There are people who do not absorb nutrients properly.
This may be due to inflammation in the small intestine.
Celiac's/gluten intolerance or Crohn's may be the culprit.

The loading dose of 50,000IU given by RX is D2. The reason for this is that there is no RX version of D3. D2 is inferior and there are places selling 50,000 IU OTC D3. I cannot verify however if these are gelcaps/liquigels.
I myself use NOW brand 2000IU and I take two of these.

There is a warning that extended use of very high D intake may result in loss of calcium thru the urine and those prone to kidney stones may have a problem.
This study on MS patients at very high levels did not show
this effect on calcium:
http://www.ncbi.nlm.nih.gov/pubmed/1...ubmed_RVDocSum
This is the full article:
http://www.ajcn.org/cgi/content/full/86/3/645

On the other hand there is a study out that showed no adverse effects of D3 on normal volunteers at levels of 10,000 IU daily.
http://www.rejuvenation-science.com/...per-limit.html

So there may be reasons that testing shows low levels.
Test errors
use of drugs that deplete D3
use of dosage form that does not dissolve
poor absorption at the intestinal level
Body using it up too quickly

All the the D information is still relatively new. And because of this I think confusion will be common, and many questions still will be forthcoming.