Serum levels of insulin-like growth factor I and insulin-like growth factor-binding protein 1 correlate with serum free testosterone and sex hormone binding globulin levels in healthy young and middle-aged men

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.