“D3 spontaneously isomerizes to vitamin D 3 (cholecalciferol) in a antarafacial sigmatropic [1,7] hydride shift. At room temperature, the transformation of previtamin D3 to vitamin D3 takes about 12 days to complete.[88]
Whether it is made in the skin or ingested, cholecalciferol is hydroxylated in the liver at position 25 (upper right of the molecule) to form 25-hydroxycholecalciferol (calcidiol or 25(OH)D). This reaction is catalyzed by the microsomal enzyme vitamin D 25-hydroxylase,[133] which is produced by hepatocytes. Once made, the product is released into the plasma, where it is bound to an α-globulin, vitamin D binding protein.[134]”
Without conversion, our supplemented cholecalciferol (d3) doesn’t convert to 25(OH) which is what’s tested. Same applies to sun exposure.