Vitamin D1, 25 Dihydroxy

Vitamin

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Assesses 25-hydroxyvitamin D levels to determine deficiency, affecting bone strength and immune function.

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Vitamin D 1,25 Dihydroxy - Comprehensive Medical Test Guide

Why is it done?
- Measures the active form of Vitamin D (calcitriol or 1,25-dihydroxyvitamin D3) in the blood, which is the most biologically potent metabolite responsible for calcium and phosphate homeostasis
- Evaluates kidney function and its ability to convert 25-hydroxyvitamin D to its active form
- Diagnoses abnormalities in calcium and phosphate metabolism
- Investigates hypercalcemia or hypocalcemia of unclear etiology
- Monitors patients with chronic kidney disease, hyperparathyroidism, or granulomatous diseases
- Assesses complications related to renal failure including secondary hyperparathyroidism and osteodystrophy
- Investigates rickets or osteomalacia when primary Vitamin D deficiency has been ruled out
- Evaluates conditions with abnormal production of calcitriol such as sarcoidosis, tuberculosis, or other granulomatous diseases
Normal Range
- Reference Range (Adults): 19.6 - 54.3 pg/mL (or 47 - 144 pmol/L)
- Units of Measurement: Picograms per milliliter (pg/mL) or picomoles per liter (pmol/L)
- Normal Result Interpretation: Indicates adequate kidney function and normal calcium-phosphate metabolism; suggests active vitamin D production is appropriate for body's needs
- High Result (>54.3 pg/mL): May indicate excessive production of calcitriol, often seen in granulomatous diseases, primary hyperparathyroidism, or lymphomas; can lead to hypercalcemia
- Low Result (<19.6 pg/mL): May indicate kidney disease with impaired activation of vitamin D, secondary hyperparathyroidism, hypoparathyroidism, or pseudo-hypoparathyroidism
- Note: Reference ranges may vary slightly between laboratories; always consult the specific lab's reference intervals for accurate interpretation
Interpretation
- Elevated Calcitriol (>54.3 pg/mL):
  - Excessive PTH stimulation (primary or tertiary hyperparathyroidism)
  - Granulomatous diseases (sarcoidosis, tuberculosis, coccidioidomycosis, histoplasmosis, blastomycosis)
  - Lymphomas (particularly Hodgkin's lymphoma)
  - Excessive vitamin D supplementation
  - Pregnancy-related elevation
- Decreased Calcitriol (<19.6 pg/mL):
  - Chronic kidney disease (most common cause) with reduced 1-alpha hydroxylase activity
  - Hypoparathyroidism or pseudo-hypoparathyroidism (inadequate PTH signaling)
  - Vitamin D deficiency (25-hydroxyvitamin D insufficiency)
  - Hereditary vitamin D-dependent rickets (Type 1 or Type 2)
  - Oncogenic osteomalacia (fibroblast growth factor 23-producing tumors)
  - Phosphate depletion
- Factors Affecting Results:
  - Parathyroid hormone (PTH) levels - primary regulator of calcitriol production
  - Serum phosphate and calcium levels - influence 1-alpha hydroxylase activity
  - Renal function - glomerular filtration rate critically affects conversion
  - Circulating 25-hydroxyvitamin D levels - substrate for activation
  - Medications affecting vitamin D metabolism (thiazides, corticosteroids, anticonvulsants)
  - Time of day and season - seasonal variation may occur
Associated Organs
- Primary Organ Systems Involved:
  - Kidneys - primary site of 1-alpha hydroxylase enzyme that converts 25-hydroxyvitamin D to active calcitriol
  - Parathyroid glands - produce PTH which stimulates renal conversion of vitamin D
  - Small intestine - target organ for calcium and phosphate absorption regulated by calcitriol
  - Bones - target organ affected by calcitriol-mediated calcium and phosphate homeostasis
  - Immune system - calcitriol affects T cell regulation and immune responses
- Common Associated Medical Conditions:
  - Chronic kidney disease (CKD) - leading cause of decreased calcitriol
  - Secondary hyperparathyroidism - develops from chronic kidney disease
  - Hypoparathyroidism - inadequate PTH production or action
  - Pseudo-hypoparathyroidism - end-organ PTH resistance
  - Rickets and osteomalacia - abnormal bone mineralization
  - Sarcoidosis and other granulomatous diseases - unregulated calcitriol production
  - Lymphomas (Hodgkin's) - produce calcitriol extrarenally
  - Primary hyperparathyroidism - excessive PTH stimulation
  - Tertiary hyperparathyroidism - post-transplant PTH dysregulation
- Potential Complications and Risks Associated with Abnormal Results:
  - Elevated Calcitriol Complications: Hypercalcemia, hypercalciuria, nephrolithiasis, tissue calcification, arrhythmias, confusion, polyuria, polydipsia
  - Low Calcitriol Complications: Hypocalcemia, hyperphosphatemia, secondary hyperparathyroidism, renal osteodystrophy, osteomalacia, increased fracture risk, tetany, seizures
  - Progressive renal disease - may worsen from untreated mineral disorders
  - Cardiovascular complications - from chronic hypercalcemia or mineral imbalances
  - Bone disease progression - renal osteodystrophy may become irreversible
Follow-up Tests
- Complementary Tests to Order with 1,25-Dihydroxyvitamin D:
  - 25-Hydroxyvitamin D (calcifediol) - assessment of overall vitamin D status and storage form
  - Parathyroid hormone (PTH) - primary regulator of calcitriol production
  - Serum calcium (total and ionized) - primary target affected by calcitriol
  - Serum phosphate - regulated by calcitriol; inverse relationship with PTH
  - Serum creatinine and estimated glomerular filtration rate (eGFR) - assess kidney function
  - Alkaline phosphatase and bone-specific alkaline phosphatase - assess bone turnover
  - 24-hour urinary calcium - evaluate hypercalciuria risk
- Additional Testing Based on Clinical Presentation:
  - Elevated Calcitriol Findings: ACE level and chest imaging for granulomatous disease; lymphoma workup if indicated; imaging for tissue calcification
  - Low Calcitriol Findings: Assess for kidney disease progression; genetic testing for hereditary vitamin D-dependent rickets if applicable
- Monitoring Frequency:
  - Chronic kidney disease patients: Every 3-6 months during CKD stages 3b-5, more frequently if on treatment
  - Dialysis patients: Monthly monitoring of serum calcium, phosphate, and PTH; calcitriol testing as clinically indicated
  - After renal transplantation: Monitor during immediate post-transplant period and during any allograft dysfunction
  - Granulomatous disease patients: Based on disease activity and clinical course
- Imaging Studies to Consider:
  - Bone mineral density (DEXA scan) - assess for osteoporosis or osteomalacia
  - Chest imaging - if granulomatous disease suspected
  - Renal ultrasound - assess for nephrolithiasis if hypercalciuria present
Fasting Required?
- Fasting Requirement: No
- Food and Fluid Intake: Patient may eat and drink normally before blood draw; no dietary restrictions necessary
- Medications to Continue or Hold:
  - Continue all regular medications unless specifically instructed otherwise by physician
  - Active vitamin D analogs (calcitriol, alfacalcidol, paricalcitol) should not be discontinued unless instructed
  - Vitamin D supplementation should continue unless directed otherwise
  - Corticosteroids that may affect vitamin D metabolism should be noted but not routinely discontinued
- Other Patient Preparation Instructions:
  - No special preparation is required for this test
  - Patient should wear comfortable, loose-fitting clothing to facilitate blood draw
  - Inform healthcare provider of any recent procedures or significant stress that may affect vitamin D metabolism
  - Report any symptoms of hypercalcemia (nausea, vomiting, polyuria, confusion) or hypocalcemia (numbness, tingling, muscle cramps) before testing
  - Sample type: Serum (blood drawn via venipuncture into appropriate collection tube)
  - Timing: No specific time of day required; minimal diurnal variation exists for this test

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