What Does Vitamin D 1,25 Check?
A 1,25-dihydroxyvitamin D [1,25(OH)₂D] test measures the active form of vitamin D and is primarily used to evaluate disorders of calcium metabolism, particularly in conditions like sarcoidosis, chronic kidney disease, and suspected vitamin D toxicity—not for routine vitamin D status assessment.
Primary Clinical Indications
Disorders of Calcium and Bone Metabolism
- 1,25(OH)₂D testing is most useful for evaluating acquired and inborn errors in the conversion of 25(OH)D to 1,25(OH)₂D, which affect calcium and bone metabolism 1
- The test helps diagnose conditions where the kidney's ability to produce active vitamin D is impaired or dysregulated 1
Sarcoidosis-Related Hypercalcemia
- In sarcoidosis patients, 1,25(OH)₂D levels should be measured alongside 25(OH)D when assessing vitamin D metabolism, particularly before considering vitamin D replacement 2
- Granulomatous macrophages in sarcoidosis produce excess 1α-hydroxylase, converting 25(OH)D to 1,25(OH)₂D, leading to hypercalcemia 2
- Approximately 11% of sarcoidosis patients have elevated 1,25(OH)₂D levels, while 84% have low 25(OH)D levels 2
- Patients with a history of hypercalcemia show relatively higher 1,25(OH)₂D levels compared to their 25(OH)D levels 2
Chronic Kidney Disease
- In CKD patients on vitamin D therapy, monitoring should include serum calcium and phosphorus every 3 months in addition to periodic 25(OH)D monitoring 3
- The kidneys perform the second hydroxylation step to produce 1,25(OH)₂D, making this test valuable when renal function is compromised 4
What 1,25(OH)₂D Does NOT Check
Not for Routine Vitamin D Status
- 25-hydroxyvitamin D [25(OH)D] is the standard test for assessing overall vitamin D status, not 1,25(OH)₂D 3, 5, 1
- The USPSTF does not recommend routine vitamin D screening in asymptomatic adults, and when testing is done, 25(OH)D is the appropriate test 2
- 25(OH)D has a longer half-life and better reflects total body vitamin D stores from both dietary intake and sunlight exposure 1
Key Physiological Distinctions
Binding and Activity Differences
- 1,25(OH)₂D has higher affinity for the vitamin D receptor (VDR) than 25(OH)D, making it the most biologically active metabolite 5
- However, 25(OH)D has higher affinity for vitamin D binding protein (VDBP), making it more stable in circulation 5
- There is a 10-24 hour time lag between vitamin D administration and biological action due to the necessary hepatic and renal conversions 4
Regulation
- Parathyroid hormone regulates the kidney's conversion of 25(OH)D to 1,25(OH)₂D 4
- This tight regulation means 1,25(OH)₂D levels can remain normal even when vitamin D stores (25(OH)D) are depleted 1
Clinical Scenarios Requiring 1,25(OH)₂D Testing
Vitamin D Toxicity
- In vitamin D toxicity, free 1,25(OH)₂D levels are often elevated even when total 1,25(OH)₂D appears normal or only mildly elevated 6
- Elevated free 1,25(OH)₂D may play a role in the pathogenesis of hypercalcemia in vitamin D toxicity 6
Hypercalcemia Workup
- When hypercalcemia is present with suspected granulomatous disease or other disorders of vitamin D metabolism 2
- Primary hyperparathyroidism patients show elevated 1,25(OH)₂D levels (mean 198 pmol/L, range 130-299) 7
Renal Disease
- Chronic renal failure patients have markedly reduced levels (mean 11 pmol/L, range 3-36) due to impaired renal hydroxylation 7
Important Caveats
Assay Considerations
- Cross-reactivity with 1,25-dihydroxyvitamin D₂ is approximately 79% in some assays 7
- The detection limit is typically around 5 pmol/L 7
- Minimum sample volume required is usually 300 μL 7