A 25(OH)D level of less than 32 ng/mL is considered vitamin D insufficient. A 25(OH)D level of less than 20 ng/mL has been used to define vitamin D deficiency. Intestinal calcium absorption is optimized at levels of more than 32 ng/mL. Parathyroid hormone levels start to rise at 25(OH)D levels less than 31 ng/mL, which is another marker for vitamin D insufficiency.
For more on the workup of vitamin D deficiency, read here.
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Cite this: Fast Five Quiz: Can You Identify and Prevent Vitamin D Insufficiency? - Medscape - Jun 04, 2014.