A: 25-hydroxyvitamin D is the barometer for vitamin D status. According to the author of this article, most commercial assays for 25-hydroxyvitamin D are good for detecting vitamin D deficiency. The 1,25-dihydroxyvitamin D assay should never be used for detecting vitamin D deficiency because levels will be normal or even elevated as a result of secondary hyperparathyroidism. The author does warn that because the 25-hydroxyvitamin D assay is costly and may not always be available, providing children and adults with adequate amounts of the vitamin. He believes approximately 800 IU of vitamin D3 per day or its equivalent should guarantee vitamin D sufficiency unless there are mitigating circumstances such as those listed in this table:
Q: What are the causes of vitamin D deficiency?
A: Causes of vitamin D deficiency include reduced skin synthesis (caused by sunscreen use, increased skin pigment, or season and latitude of exposure), malabsorption (Crohn's disease, celiac disease, and bypass surgery), steroids (increased catabolism), breast feeding, liver failure, nephrotic syndrome, and chronic kidney disease. Other causes of vitamin D deficiency include inherited disorders (pseudovitamin D deficiency rickets), primary hyperparathyroidism and granulomatous disorders. See table for complete list.
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