Obstructive sleep apnoea (OSA) is reported to have effects on a number of hormone systems including the hypothalamo-pituitary axis. We aimed to determine the impact of OSA severity on IGF-I levels.This is a prospective cohort study performed between November 2014 and May 2017. IGF-I was measured on serum samples and data collected on demographics, BMI, and parameters of OSA.611 participants were recruited (202 female, 53.5±12.5 years; mean BMI 36.2±8.0kg/m2 ). 26.2% had mild OSA; 27.3% moderate OSA; and 44.5% severe OSA. 15.2% of IGF-I values were below the age-related reference range. Increasing BMI correlated with greater AHI (r=0.28, p<0.001), ODI (r=0.30, p<0.001), severity of OSA (r=0.17, p<0.001), duration with oxygen saturation (SaO2 ) <90% (r=0.29, p=0.001) and reduced median SaO2 levels (r=0.19, p<0.001). IGF-I levels correlated negatively with age (r=-0.13, p=0.001), BMI (r=-0.16, p<0.001), diabetes (r=-0.108, p=0.009), AHI (r=0-.10, p=0.043) and severity of OSA (r=-0.10, p=0.013). No association of IGF-I was observed with ODI, median SaO2 levels, or duration of SaO2 <90%. Regression analyses were used to examine determinants of IGF-I, all of which contained the independent variables of age, gender and BMI. All models showed IGF-I to be predicted by age and BMI (p<0.05), however, none of the parameters of OSA were significant within these models.IGF-I levels in OSA are dependent on age and BMI, however, no additional effect of any OSA parameter was observed, supporting the hypothesis that OSA effects on IGF-I are indirect through concomitant body composition and metabolic parameters.