How is vitamin D deficiency diagnosed biochemically?

• A. Low 25-hydroxyvitamin D with secondary hyperparathyroidism; PTH elevated.
• B. Low 1,25-dihydroxyvitamin D with normal PTH.
• C. High 25-hydroxyvitamin D with elevated PTH.
• D. Low calcium with normal phosphate.

Answer: (B)

The key idea is that vitamin D status affects both its circulating metabolites and the parathyroid response, and the active form is tightly linked to substrate availability and PTH-driven activation. In vitamin D deficiency, the substrate 25-hydroxyvitamin D is reduced, so the kidney’s production of the active form, 1,25-dihydroxyvitamin D, falls as well. If calcium levels are still being maintained, the parathyroid glands may not yet ramp up, leaving PTH in the normal range. That combination—low 1,25-dihydroxyvitamin D with a normal PTH—can therefore reflect deficiency in its earlier stage before secondary hyperparathyroidism develops. It highlights why 1,25-dihydroxyvitamin D alone isn’t the most sensitive marker for deficiency, since PTH and calcium can keep the active vitamin D level from dropping immediately. In practice, measuring 25-hydroxyvitamin D is the most reliable way to assess vitamin D stores, while 1,25-dihydroxyvitamin D can be low in deficiency but is not the best sole indicator. The other patterns described would be more characteristic of established secondary hyperparathyroidism or non–vitamin D–related abnormalities, so they don’t fit as well for diagnosing deficiency.