It might also be worth mentioning that measuring 25-hydroxy is also an approximation at best. That still has to be converted to 1,25-dihydroxyvitamin by the kidneys. People with high insulin resistance (diabetics, pre-diabetics) or kidney damage tend to have problems converting 25-hydroxy. Measuring 25-hydroxy is primarily used for toxicity and even that has variables that are difficult to measure, so a statistical baseline is applied.