IGF-1 Reference Ranges and Z-Score Interpretation
For adults with growth hormone deficiency (GHD), target IGF-1 levels in the upper half of the age-adjusted reference range during treatment, while for diagnosis, a low IGF-1 (below the age-adjusted normal range) supports but does not confirm GHD 1.
Diagnostic Thresholds
Adults
- Low IGF-1 alone does NOT exclude or confirm GHD - provocative testing remains mandatory when IGF-1 is normal 1
- A low IGF-1 (<84 ng/mL in some assays) combined with 3+ other pituitary hormone deficiencies is highly specific for GHD and may obviate provocative testing 1
- IGF-1 below the age-adjusted normal range helps distinguish true GHD from blunted GH responses in patients with elevated BMI 1
Children
- An IGF-1 Z-score of -2.41 or lower provides optimal discrimination between GHD and idiopathic short stature (sensitivity 74.3%, specificity 89.2%) 2
- Children with permanent GHD typically show IGF-1 Z-scores around -1.76 (range -2.25 to -0.43), compared to -0.84 to -1.16 in those with temporary or no growth impairment 3
Treatment Targets
GH Replacement Dosing Strategy
Maintain IGF-1 levels below the upper limit of normal during treatment 4. The Endocrine Society guidelines specifically recommend:
Target range: Upper half of age-adjusted reference range (though no published studies provide specific guidance on this target) 1
Starting doses by age:
- Ages 30-60 years: 200-300 μg/day
- Age ≥60 years: 100-200 μg/day
- Age <30 years: 400-500 μg/day
- Transitioning from pediatric treatment: even higher doses may be appropriate 1
Dose titration: Increase by 100-200 μg every 1-2 months based on clinical response, side effects, and IGF-1 levels 1
Gender and Estrogen Considerations
Women on oral estrogen require substantially higher GH doses due to SOCS2-mediated hepatic GH resistance (approximately 85% of circulating IGF-1 is liver-derived) 1. Women on transdermal estrogen may not require dose adjustments 1. When women discontinue oral estrogen or switch to transdermal, GH doses must be lowered 1.
Critical Interpretation Caveats
Age-Related Decline
GH secretion and IGF-1 levels decrease with age, requiring lower doses in older patients who have increased susceptibility to side effects 1. Different cut-points may be needed for distinct age populations (e.g., higher thresholds for adolescents/young adults vs. middle-aged vs. elderly >65 years) 1.
Assay-Dependent Variability
Major pitfall: Different IGF-1 immunoassays produce markedly different results even when following consensus guidelines 5. Upper limits of reference intervals vary substantially between assays, though lower limits are more consistent 5. Pairwise concordance between six commercial assays ranged only 0.38-0.70 5. Always use the same assay method for longitudinal monitoring and apply method-specific reference ranges 6.
Pubertal Confounders in Children
Near puberty, IGF-1 interpretation becomes particularly challenging 7:
- 15.5% of girls with Tanner stage 1 breasts had pubertal estradiol levels (≥25 pmol/L)
- 15.7% of boys with testes <4 mL had pubertal testosterone levels (≥0.47 nmol/L)
- Sex steroid levels inversely correlate with IGF-1 SDS, leading to potential overestimation of IGF-1 when sex steroids are low despite clinical pubertal signs 7
- Girls peak at age 13 years (median 321.7 ng/mL), boys at age 15 years (median 318.9 ng/mL) 8
BMI Effects
Increased BMI blunts GH responses to stimulation but paradoxically increases IGF-1 levels even in documented GHD 1. This makes low IGF-1 particularly useful for distinguishing true GHD from obesity-related blunted GH responses.
Monitoring During Treatment
Assess IGF-1 levels at 1-2 month intervals during dose titration, then semiannually 1. Each assessment should include:
- Clinical evaluation for side effects
- IGF-1 measurement (maintain in age-adjusted reference range, targeting upper half)
- Annual lipid profile and fasting glucose
- Reassessment of thyroid hormone and HPA axis as GH affects these systems 1
Normal IGF-1 does not exclude GHD diagnosis but makes provocative testing mandatory 1. Conversely, having 3+ other pituitary hormone deficiencies with low IGF-1 makes provocative testing optional 1.