Unit Converter
Vitamin D3, 1,25-dihydroxyvitamin D (Calcitriol)
(1,25-OH₂ Vitamin D - Active Hormone; Regulates Calcium, Phosphate & Parathyroid Physiology)
Synonyms
- 1,25-Dihydroxyvitamin D
- 1,25-OH₂ D
- Calcitriol
- Active vitamin D
- 1α,25-dihydroxycholecalciferol
- 1,25-OH₂ D₃
- Hormonal form of vitamin D
- “Active D3”
Units of Measurement
- pmol/L
- ng/L
- ng/dL
- ng/100 mL
- ng%
- pg/mL
Unit Conversions
Molecular weight of 1,25-dihydroxyvitamin D₃ ≈ 416.65 g/mol
pmol/L ↔ pg/mL
1 pmol/L=0.416 pg/mL1\ \text{pmol/L} = 0.416\ \text{pg/mL}1 pmol/L=0.416 pg/mL 1 pg/mL=2.40 pmol/L1\ \text{pg/mL} = 2.40\ \text{pmol/L}1 pg/mL=2.40 pmol/L
ng/L ↔ pg/mL
1 ng/L=1 pg/mL1\ \text{ng/L} = 1\ \text{pg/mL}1 ng/L=1 pg/mL
ng/dL → pg/mL
1 ng/dL=10 pg/mL1\ \text{ng/dL} = 10\ \text{pg/mL}1 ng/dL=10 pg/mL
ng/100 mL = ng% = ng/dL
Description
1,25-Dihydroxyvitamin D (Calcitriol) is the active hormonal form of vitamin D.
It is produced from 25-OH Vitamin D (calcidiol) by 1-alpha hydroxylation in:
- Kidneys (primary site)
- Macrophages & extrarenal tissues (immune activation)
Calcitriol regulates:
- Calcium absorption
- Phosphate homeostasis
- Bone turnover
- PTH suppression
Important:
Calcitriol is NOT a good indicator of vitamin D stores.
It may remain normal or even elevated in vitamin D deficiency due to secondary hyperparathyroidism.
Physiological Role
1. Calcium Absorption
Upregulates TRPV6 channels, calbindin → increases Ca²⁺ absorption.
2. Phosphate Absorption
Increases Na/P co-transporter activity.
3. Bone Remodeling
Works with PTH:
- Promotes osteoblast differentiation
- Regulates osteoclastogenesis indirectly
4. Parathyroid Regulation
Suppresses PTH gene transcription.
5. Renal Mineral Homeostasis
Modulates calcium/phosphate reabsorption.
6. Immune Modulation
Extrarenal calcitriol production affects:
- Antimicrobial response
- Autoimmune modulation
Clinical Significance
LOW 1,25-OH₂ D (Calcitriol Deficiency)
Causes
- Chronic Kidney Disease (CKD)
(Reduced 1-α hydroxylase activity; hallmark of CKD–MBD) - Hypoparathyroidism (low PTH → low calcitriol)
- Severe Vitamin D deficiency (also decreases substrate)
- Hereditary vitamin D–dependent rickets (VDDR types 1 & 2)
- Chronic liver disease
- Tumor-induced osteomalacia (FGF23 excess → suppresses 1-α hydroxylase)
- Medications: ketoconazole, anticonvulsants
Clinical Features
- Hypocalcemia
- Hypophosphatemia
- Muscle cramps
- Seizures (neonates)
- Rickets / osteomalacia
- Tetany
HIGH 1,25-OH₂ D (Calcitriol Excess)
Causes
- Primary hyperparathyroidism
- Granulomatous diseases:
- Sarcoidosis
- Tuberculosis
- Crohn’s disease
(macrophages produce calcitriol autonomously)
- Sarcoidosis
- Lymphoma
- Vitamin D intoxication (late finding)
- CYP24A1 mutation (reduced degradation)
Clinical Features
- Hypercalcemia
- Suppressed PTH
- Nephrocalcinosis
- Kidney stones
- Polyuria / polydipsia
Reference Intervals
(Tietz 8E + Mayo + ARUP + Endocrine Society)
1,25-OH₂ Vitamin D (Calcitriol)
| Age | pg/mL | pmol/L |
| Adults | 18 – 72 pg/mL | 43 – 173 pmol/L |
| Children | Slightly higher physiological levels |
Deficiency
- <18 pg/mL (<43 pmol/L)
Excess
- >72 pg/mL (>173 pmol/L)
Diagnostic Uses
1. Renal Osteodystrophy / CKD–MBD Evaluation
Primary test when CKD suspected.
2. Investigation of Hypercalcemia
Differentiate:
- PTH-mediated
- Non-PTH granulomatous causes
3. Workup of
Hypocalcemia
Determine PTH-vitamin D axis involvement.
4. Assessment of Rickets / Osteomalacia
Especially in:
- Vitamin D-dependent rickets
- Phosphate-wasting disorders (FGF23 excess)
5. Immunologic Diseases
Granulomatous disorders increase calcitriol.
Analytical Notes
- Specimen: serum (preferred)
- Timing: no fasting needed
- Method: LC-MS/MS is most accurate
- Calcitriol is present in picogram quantities - sensitive analytical methods required
- Very sensitive to:
- Assay interference
- Renal function
- PTH status
- Assay interference
Do NOT use this test to assess general Vitamin D status.
Use 25-OH D for routine deficiency screening.
Clinical Pearls
- In CKD, 25-OH D may be normal, but calcitriol is low → secondary hyperparathyroidism.
- Granulomatous diseases cause hypercalcemia with high calcitriol and low PTH.
- High calcitriol + high calcium + low PTH → think sarcoidosis or lymphoma.
- In nutritional deficiency, calcitriol may be normal early but falls when 25-OH D substrate is extremely low.
- Neonates with seizures may have calcitriol-dependent rickets.
Interesting Fact
Calcitriol acts like a steroid hormone, binding to nuclear vitamin D receptors (VDR) across >36 tissues and influencing over 200 genes.
References
NIH Office of Dietary Supplements - Vitamin D Pathways
Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Vitamin D Metabolites
Endocrine Society Clinical Practice Guidelines - Vitamin D, 2024
KDIGO CKD-MBD Guidelines
Mayo Clinic Laboratories - Calcitriol (1,25-OH₂ D)
ARUP Consult - Vitamin D Testing