1,25(OH)2D is the best blood test for determining adequacy of vitamin D stores in the body.

• A. True
• B. False
• C. Not sure
• D. Inconclusive

Answer: (B)

The test of vitamin D status is best answered by measuring 25-hydroxyvitamin D in the blood. This form reflects how much vitamin D is available from sun exposure, diet, and supplements, and it represents the body’s overall stores because it has a relatively long circulating half-life of about two to three weeks.

In contrast, the active form, 1,25-dihydroxyvitamin D, is produced by the kidneys under tight hormonal control (primarily by parathyroid hormone) and responds to calcium and phosphate levels. Its levels can be normal or even elevated in early deficiency and can decline only later, if kidney function worsens or stores are severely depleted. It also has a short half-life, so it does not accurately reflect total vitamin D stores in the body.

So the statement is false: 25(OH)D is the best measure of vitamin D stores, not 1,25(OH)2D.