25D

25D is shorthand for 25-hydroxyvitamin D (25(OH)D), the major circulating form of vitamin D and the standard clinical measure of vitamin D status.

It is produced in the liver from vitamin D obtained through sunlight exposure, diet, or supplements. It exists in two main forms: 25-hydroxyvitamin D2 (from ergocalciferol) and 25-hydroxyvitamin D3 (from cholecalciferol). The circulating pool reflects total intake and endogenous synthesis. It is the precursor to the active hormone 1,25-dihydroxyvitamin D, formed in the kidney and other tissues.

Measurement uses serum or plasma levels, reported in ng/mL (nanograms per milliliter) or nmol/L. Common reference ranges: deficient <20 ng/mL (50 nmol/L), insufficient 20-29 ng/mL (50-72.5 nmol/L), sufficient ≥30 ng/mL (≥75 nmol/L). Note that guidelines vary.

Clinical relevance: assessment of vitamin D status in bone health, osteoporosis risk, and potential immune function roles. Levels can be influenced by age, obesity, skin pigmentation, exposure to sunlight, geographic latitude, and certain medications.

Analytical considerations: testing methods include immunoassays and LC-MS/MS; not all assays distinguish between D2 and D3, and results can vary by method. High levels of 25D rarely cause toxicity, but excessive supplementation can lead to hypercalcemia.

Disambiguation: The designation "25D" can also appear in other contexts, including as shorthand for other compounds or substances; in medical literature, it most commonly denotes 25-hydroxyvitamin D.