Serum levels of which metabolite indicate sufficient vitamin D status?

• A. 25-hydroxyvitamin D (25(OH)D) above about 30 ng/mL
• B. 1,25-dihydroxyvitamin D
• C. Vitamin D2
• D. Parathyroid hormone

Answer: (A)

Measuring vitamin D status is best done with serum 25-hydroxyvitamin D because it reflects the total amount of vitamin D stored in the body from both sun exposure and dietary sources, and it remains in the bloodstream longer than the active form. A level around or above 30 ng/mL is commonly used as a threshold for sufficiency, since this level is associated with better bone health and lower risk of deficiency-related problems.

The active form, 1,25-dihydroxyvitamin D, is tightly regulated and can remain normal or even be high in early deficiency due to secondary hormonal responses; its level does not reliably indicate overall vitamin D stores. Vitamin D2 is just one dietary form and does not by itself indicate overall vitamin D status. Parathyroid hormone responds to calcium and vitamin D balance but is influenced by many factors and does not directly measure vitamin D stores.