Now there's evidence that stress, fatigue, and depression are connected to low serum levels of vitamin D, in addition to known immunity, illness and disease issues.
The more up-to-date integrative or holistic MDs and naturopaths recognize 25-hydroxy-vitamin D or 25(OH)D3 test serum level should be in the 50 to 80 ng/ml (nanograms per milliliter) for optimum health. 
The 25(OH)D3 test is the only test that will accurately assess your vitamin D levels. Yet, sometimes doctors order the 1,25-dihydroxy-vitamin D test. Often, people whose vitamin D serum levels are actually low will read high on that test and be led to believe that all is well and be misdiagnosed.