Supplementary Material for: Predictors of Insulin-Like Growth Factor-I Responses to Growth Hormone Replacement in Young Adults with Growth Hormone Deficiency

<b><i>Background/Aims:</i></b> Physiological growth hormone (GH) secretion and insulin-like growth factor-I (IGF-I) levels are greater in young compared to older adults. We evaluated IGF-I levels and predictors of IGF-I responses in young adults on GH replacement. <b><i>Design:</i></b> From the KIMS database, 310 young adults (age 15-26 years) with severe GH deficiency related to childhood-onset disease and commenced on ‘adult GH replacement' were identified. ‘IGF-I responses' were estimated from first-year increments in IGF-I standard deviation scores (SDS) and adjusted for GH dose. Body composition was assessed by bioimpedance in 143 patients. <b><i>Results:</i></b> IGF-I levels increased markedly from baseline to 1 year of replacement (-3.75 ± 1.94 vs. -1.36 ± 1.86 SDS, p &lt; 0.0001), but remained low compared to normative data despite dose titration. In multivariate models, IGF-I responses were positively associated with age [B (SE) SDS/(mg/m²); 0.52 (0.15), p = 0.0007] and BMI SDS [1.06 (0.25), p &lt; 0.0001] and inversely associated with female gender [-4.45 (0.79), p &lt; 0.0001] and baseline IGF-I SDS [-1.44 (0.20), p &lt; 0.0001]. IGF-I responses were positively associated with first-year increases in lean body mass (r = 0.19, p = 0.003) and haemoglobin A1c (r = 0.15, p = 0.031). <b><i>Conclusions:</i></b> Low IGF-I levels in young adults on treatment may reflect suboptimal GH replacement. Identification of predictors for IGF-I responses could lead to a more appropriate replacement strategy. Association between IGF-I responses and lean body mass suggests that maintaining age-appropriate IGF-I levels is important during therapy.