PTH and Vitamin D Testing in Pediatrics
Key Points
- Essential markers for calcium homeostasis and bone metabolism
- Interdependent relationship between PTH and Vitamin D
- Age-specific reference ranges apply
- Critical for growth and skeletal development
Physiological Overview
- Calcium Homeostasis System:
- PTH: Primary calcium-regulating hormone
- Vitamin D: Essential for calcium absorption
- Calcitonin: Counter-regulatory hormone
- Feedback Mechanisms:
- Calcium-sensing receptor regulation
- 1α-hydroxylase activity
- Bone remodeling processes
Vitamin D Assessment
Forms and Measurement
- 25-hydroxyvitamin D (25-OH-D):
- Primary circulating form
- Best indicator of vitamin D status
- Half-life: 2-3 weeks
- Normal range: 20-50 ng/mL (50-125 nmol/L)
- 1,25-dihydroxyvitamin D:
- Active hormonal form
- Limited clinical utility
- Half-life: 4-6 hours
- Normal range: 20-45 pg/mL
Status Classification
- Deficiency: <20 ng/mL
- Severe: <10 ng/mL
- Moderate: 10-15 ng/mL
- Mild: 15-20 ng/mL
- Insufficiency: 21-29 ng/mL
- Sufficiency: 30-100 ng/mL
- Toxicity: >100 ng/mL
PTH Evaluation
Measurement Considerations
- Sample Collection:
- Early morning sampling preferred
- Fasting state recommended
- Special handling requirements
- Stability considerations
- Reference Ranges:
- Intact PTH: 10-65 pg/mL
- Age-specific variations
- Method-dependent results
Analytical Aspects
- Assay Types:
- Third-generation intact PTH
- Bio-intact PTH (1-84)
- C-terminal PTH fragments
- Interfering Factors:
- Renal function
- Time of day
- Exercise status
- Dietary calcium intake
Clinical Applications
Primary Indications
- Growth Assessment:
- Failure to thrive
- Growth velocity changes
- Skeletal deformities
- Disease Monitoring:
- Rickets evaluation
- Chronic kidney disease
- Malabsorption syndromes
- Endocrine disorders
Screening Recommendations
- High-Risk Groups:
- Premature infants
- Dark-skinned children
- Obesity
- Limited sun exposure
- Chronic Conditions:
- Inflammatory bowel disease
- Celiac disease
- Cystic fibrosis
- Epilepsy (on anticonvulsants)
Integrated Interpretation
Pattern Recognition
- Primary Hyperparathyroidism:
- ↑PTH, ↑/Normal Calcium
- Normal/↓Vitamin D
- Secondary Hyperparathyroidism:
- ↑PTH, ↓/Normal Calcium
- ↓Vitamin D common
- Vitamin D Deficiency:
Monitoring Considerations
- Treatment Response:
- Vitamin D supplementation
- Calcium supplementation
- Therapeutic interventions
- Follow-up Intervals:
- Severe deficiency: 3 months
- Mild-moderate deficiency: 6 months
- Maintenance: Annually
Further Reading
