Free/dissociable insulin-like growth factor (IGF)-I, not total IGF-I, correlates with growth response during growth hormone treatment in children born small for gestational age.

Context: IGF-I plays an important role in pre- and postnatal growth. Its serum levels are regulated by metabolic and genetic factors. Mean total IGF-I in short, small for gestational age (SGA) children is reduced, but within the normal range. Free/dissociable IGF-I is the bioactive form of IGF-I.
Objectives: The aim of the study was to investigate changes in free IGF-I during GH treatment in short SGA children and to evaluate whether free IGF-I levels contribute to predicting first-year growth response and/or adult height. DESIGN, SETTING, AND INTERVENTION: We conducted a randomized, double-blind GH dose-response study with a GH dose of either 1 mg/m(2).d (group A) or 2 mg/m(2).d (group B). Free IGF-I, total IGF-I, and IGF binding protein (IGFBP)-3 were determined at baseline, after 1 and 5 yr, at stop, and 6 months after GH discontinuation.
Patients: We studied 73 (46 male) short SGA children (36 group A) with a baseline mean age of 7.7 (2.2) yr and a mean GH duration of 8.2 (2.1) yr.
Main Outcome Measures: Untreated SGA children had a mean free IGF-I sd score (SDS) of -0.2 (1.2), not related to total IGF-I. During GH therapy, free IGF-I significantly increased to 1.6 (0.7) SDS, as did total IGF-I and IGFBP-3 [2.0 (0.8) and 1.3 (0.9), respectively]. Multiple regression analysis showed that baseline free IGF-I and IGFBP-3 were negatively correlated with adult height SDS, whereas baseline bone age delay, target height SDS, baseline height SDS, and GH dose were positively correlated. Free IGF-I was also negatively correlated with first-year growth response.
Conclusions: Circulating baseline free IGF-I and IGFBP-3 were better predictors for adult height in GH-treated SGA children than total IGF-I, or total IGF-I to IGFBP-3 ratio. This suggests a possible role for free IGF-I measurement in predicting the effect of GH therapy in short SGA children.