Objective: Administration of testosterone has been reported to increase serum levels of IGF-I in men with isolated hypogonadotrophic hypogonadism. An inverse relation between serum IGF-I and sex hormone binding globulin (SHBG) is seen in GH deficient children. The biological action of IGF-I is thought to be influenced by binding proteins, one of which is insulin-like growth factor-binding protein-1 (IGFBP-1), which is not only a carrier protein but also actively regulates the cellular actions of IGF-I. These observations suggest associations between IGF-I, IGFBP-1, testosterone and SHBG in serum. The aim of the present study was to investigate these associations in normal healthy men.
Design and patients: The associations between the serum levels of IGF-I and IGFBP-1 on one hand, and testosterone and SHBG on the other were investigated in 38 normal healthy young and middle-aged men.
Results: Serum levels of IGF-I decreased both with increasing age (r = -0.66, P < 0.001) and increasing SHBG levels (r = -0.46, P = 0.002), but increased with increasing free testosterone (f-testosterone) (r = 0.42, P = 0.005). These associations remained after mutual simultaneous adjustments in a multiple regression analysis. IGFBP-1 did not display any significant univariate correlation with age (r = -0.25, P = 0.06) or SHBG (r = 0.18, P = 0.14), but showed a significant positive correlation with both f-testosterone (r = 0.42, P = 0.004), and total testosterone (t-testosterone) (r = 0.39, P = 0.008). In a multiple regression analysis IGFBP-1 was positively correlated with both SHBG and f-testosterone, but not with t-testosterone.
Conclusion: The present study suggests that among healthy young and middle-aged men, there is an association between serum levels of free-testosterone and SHBG on the one hand, and serum IGF-I and IGFBP-1 on the other.