Why does the laboratory assay measure 25-OH vitamin D?

25-OH vitamin D is the major storage form of vitamin D; hence it is a good indication of how much vitamin D your body has. 25-OH vitamin D is present in the blood at up to 1000-fold higher concentration compared to the active 1,25-(OH)2-vitamin D (which is generated from 25-OH vitamin D in the kidneys). 25-OH vitamin D has a half-life of 2-3 weeks vs. 4 hours for 1,25-(OH)2-vitamin D. Therefore, 25-OH vitamin D is the analyte of choice for determination of the vitamin D status.