1,25-Dihydroxyvitamin D
Overview
1,25-Dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol, is the active hormonal form of vitamin D produced by 1-alpha-hydroxylation of 25-hydroxyvitamin D in the kidney. It regulates calcium and phosphate homeostasis by promoting intestinal absorption, bone resorption, and renal reabsorption while suppressing parathyroid hormone secretion. Low levels indicate renal failure, hypoparathyroidism, or 1-alpha-hydroxylase deficiency, while high levels suggest sarcoidosis, lymphoma, or vitamin D intoxication. Unlike 25(OH)D, 1,25(OH)2D is not useful for assessing vitamin D nutritional status due to tight physiologic regulation and short half-life.
Clinical Use Cases
- Diagnosis of familial hypophosphatemic rickets and oncogenic osteomalacia.
- Evaluation of hypercalcemia of malignancy or granulomatous disease.
- Assessment of hypoparathyroidism and pseudohypoparathyroidism.
- Investigation of vitamin D-resistant rickets.
- Monitoring therapy in chronic kidney disease (with 1,25(OH)2D analogs).
Specimen Types
- Serum.
- Plasma (EDTA).
Measurement Methods
- Liquid chromatography-tandem mass spectrometry (LC-MS/MS).
- Radioimmunoassay.
- Competitive protein binding assay.
Test Preparation and Influencing Factors
- No fasting required.
- Renal function critically affects production (decreased in CKD).
- Calcium, phosphate, PTH, and fibroblast growth factor 23 tightly regulate levels.
- Anticonvulsants and rifampin induce 24-hydroxylase, lowering levels.
- Sample light-sensitive; protect from light during processing.
- Seasonal variation less pronounced than 25(OH)D.
Synonyms
- Calcitriol.
- 1,25(OH)2D.
- 1,25-Dihydroxycholecalciferol.