Classification and biochemistry: Vitamin D is a fat-soluble secosteroid prohormone produced in the skin from sunlight and obtained from the diet, then hydroxylated in the liver to 25-hydroxyvitamin D (25(OH)D), the primary circulating form measured in blood. Because 25(OH)D is relatively stable and reflects recent input and storage, it is widely used as a biochemical indicator of vitamin D availability. Biological functions and context: 25(OH)D is further activated to form a hormone that binds the vitamin D receptor to influence gene expression. Through these pathways it supports mineral balance, bone-related processes, immune cell signaling, muscle function, and cellular growth control. Concentrations are affected by sun exposure, diet, body composition, liver and kidney metabolism, and genetic variation in vitamin D‑processing enzymes and binding proteins.

Elevated 25(OH)D levels may reflect high supplemental intake, changes in vitamin D–binding protein or assay interference, or altered metabolism that increases circulating 25(OH)D; interpretation requires checking related markers such as calcium, parathyroid hormone, and renal function. Low 25(OH)D levels may indicate limited sun exposure, low dietary intake, reduced absorption, increased tissue demand, adipose sequestration, or genetic and medication effects that impair synthesis or transport. Always interpret 25(OH)D alongside the full Nutrient Zoomer panel and clinical context to identify patterns and potential drivers of altered vitamin D biology.

The Nutrient Zoomer includes 25(OH)D to quantify vitamin D availability as part of a comprehensive nutrient and genetics profile. Measuring this marker helps reveal how vitamin D status interacts with mineral balance, immune signaling, and genetic variants that affect vitamin D processing, supporting more personalized nutritional insights and patterns of nutrient demand.