Growth Hormone Level Testing
Growth Hormone Testing in Pediatrics
Growth Hormone (GH) testing is crucial in evaluating children with growth disorders. Due to the pulsatile nature of GH secretion, testing requires specific protocols and careful interpretation.
Key Points
- GH secretion is pulsatile with significant diurnal variation
- Peak secretion occurs during slow-wave sleep
- Random GH measurements have limited diagnostic value
- IGF-1 and IGFBP-3 are useful adjunct markers
GH Physiology and Secretion
Secretion Pattern
- Pulsatile secretion every 3-4 hours
- Major secretory episodes during slow-wave sleep
- Approximately 6-8 pulses per 24 hours
- Amplitude increases during puberty
Regulatory Factors
- Stimulatory:
- GHRH (Growth Hormone Releasing Hormone)
- Ghrelin
- Exercise
- Sleep
- Hypoglycemia
- Inhibitory:
- Somatostatin
- Hyperglycemia
- Free fatty acids
- IGF-1 (negative feedback)
GH Testing Methods
Baseline Measurements
- IGF-1 (Insulin-like Growth Factor 1)
- More stable than GH
- Reflects average GH secretion
- Age and sex-specific ranges
- IGFBP-3 (IGF Binding Protein 3)
- Less nutritionally dependent than IGF-1
- More stable throughout the day
- Useful in malnutrition cases
Screening Tests
- Height and weight measurements
- Growth velocity calculation
- Bone age determination
- Basic metabolic panel
- Thyroid function tests
GH Stimulation Testing
Common Stimulation Tests
- Arginine Test
- Dose: 0.5g/kg (max 30g)
- Sampling: 0, 30, 60, 90, 120 min
- Mechanism: Suppresses somatostatin
- Clonidine Test
- Dose: 0.15 mg/m²
- Sampling: 0, 30, 60, 90, 120 min
- Monitor blood pressure
- Glucagon Test
- Dose: 0.03 mg/kg (max 1mg)
- Sampling: 0, 30, 60, 90, 120, 150, 180 min
- More prolonged testing required
- Insulin Tolerance Test
- Dose: 0.05-0.1 U/kg IV
- Sampling: 0, 15, 30, 45, 60, 90 min
- Requires careful monitoring
- Contraindicated in seizure disorders
Result Interpretation
Normal Values
- Stimulation Test Response:
- Normal: Peak GH >10 ng/mL
- Partial deficiency: Peak 5-10 ng/mL
- Severe deficiency: Peak <5 ng/mL
- IGF-1 Levels:
- Age and sex-specific
- Usually reduced in GH deficiency
- May be normal in partial deficiency
Factors Affecting Results
- Obesity (false low values)
- Malnutrition
- Concurrent medications
- Stress
- Recent exercise
- Pubertal status
Clinical Applications and Considerations
Indications for Testing
- Height >2 SD below mean for age
- Growth velocity <25th percentile
- Predicted adult height significantly below target
- History of CNS tumors/radiation
- Multiple pituitary hormone deficiencies
- Neonatal hypoglycemia with micropenis
Pre-test Requirements
- Adequate nutrition status
- Euthyroid state
- Fasting (usually 6-8 hours)
- Proper hydration
- No acute illness
Monitoring Considerations
- Regular height/weight measurements
- Annual bone age assessment
- IGF-1 monitoring during treatment
- Regular assessment of treatment response
