Vitamin D, 25-Hydroxy

The Vitamin D, 25-Hydroxy test (25[OH]D; calcidiol; 25-hydroxycalciferol) is used to diagnose vitamin D deficiency and identify excessive supplementation, and to assess disorders of calcium–phosphate metabolism and skeletal disease. It is also used to monitor response to vitamin D therapy and to guide dose adjustments during replacement. Clinicians incorporate 25(OH)D measurement in the workup of hypocalcemia with abnormal parathyroid hormone concentrations, as a baseline before initiating anti‑osteoporotic treatment, and for patients with malabsorption syndromes.

Vitamin D is a fat‑soluble prohormone that regulates serum calcium, phosphorus, and magnesium homeostasis. Hepatic conversion of cholecalciferol (D3) or ergocalciferol (D2) produces 25‑hydroxyvitamin D [25(OH)D], the chief circulating form; because it circulates at higher concentrations and has a longer half‑life than the active hormone, it is the most reliable indicator of vitamin D status. The biologically active metabolite, 1,25‑dihydroxyvitamin D, is generated in the kidney. Endogenous cholecalciferol is synthesized in skin under ultraviolet B exposure, while exogenous sources include diet such as fatty fish and fish liver oils. Deficiency impairs skeletal mineralization, causing rickets in children and osteomalacia in adults, and contributes to osteoporosis risk. Individuals with malabsorption, liver disease, nephrotic syndrome, limited sun exposure, or advanced age are at increased risk. Excessive supplementation can lead to toxicity with hypercalcemia and associated organ damage. Measured 25(OH)D concentrations vary with sunlight exposure. Some assays detect small amounts of other metabolites (e.g., 24,25‑dihydroxyvitamin D), but their low physiologic levels relative to 25(OH)D do not meaningfully affect assessment of vitamin D status.