Which laboratory value is most useful to assess vitamin D status in osteoporosis evaluation?

• A. 1,25-dihydroxy vitamin D
• B. 25-hydroxy vitamin D
• C. Parathyroid hormone
• D. Serum iron

Answer: (B)

Measuring vitamin D status in osteoporosis relies on the circulating storage form, 25-hydroxy vitamin D, because it accumulates over time and reflects total vitamin D obtained from sun and diet. This form has a relatively long half-life, giving a stable snapshot of an individual’s vitamin D stores. The active form, 1,25-dihydroxy vitamin D, is tightly controlled by parathyroid hormone, calcium, and phosphate and can remain normal or even be elevated in early deficiency, so it does not reliably indicate overall vitamin D status. Parathyroid hormone changes react to calcium balance rather than directly measuring vitamin D stores, making it a downstream marker rather than a direct assessment of vitamin D status. Serum iron is not related to vitamin D status, so it isn’t useful for this purpose. Therefore, 25-hydroxy vitamin D best reflects vitamin D sufficiency or deficiency and guides osteoporosis management.