Which laboratory test best reflects total body vitamin D stores and is used to define vitamin D deficiency?

• A Serum 1,25-dihydroxyvitamin D level
• B 24-hour urinary calcium excretion
• C Serum parathyroid hormone level
• D Serum 25-hydroxyvitamin D level ✓

Explanation

Serum 25-hydroxyvitamin B is the major circulating storage form, has a half-life of several weeks, and is the standard measure of vitamin B status. Values below 20 ng/mL indicate deficiency and 20 to 30 ng/mL insufficiency. The 1,25-dihydroxyvitamin B level is misleading because it is tightly regulated by PTH and may be normal or even elevated in deficiency, while PTH rises only secondarily and is influenced by many other factors.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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