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 hypogona dism. An inverse relation between serum IGF-I and sex hormone binding globulin (SHBG) is seen in GH deficient children. The biological actio n of IGF-I is thought to be influenced by binding proteins, one of whi ch is insulin-like growth factor-binding protein-1 (IGFBP-1), which is not only a carrier protein but also actively regulates the cellular a ctions of IGF-I. These observations suggest associations between IGF-I , IGFBP-1, testosterone and SHBG in serum. The aim of the present stud y was to investigate these associations in normal healthy men. DESIGN AND PATIENTS The associations between the serum levels of IGF-I and IG FBP-1 on one hand, and testosterone and SHBG on the other were investi gated in 38 normal healthy young and middle-aged men. RESULTS Serum le vels 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 incre ased with increasing free testosterone (f-testosterone) (r = 0 . 42, P = 0 . 005). These associations remained after mutual simultaneous adj ustments in a multiple regression analysis. IGFBP-1 did not display an y 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 to tal testosterone (t-testosterone) (r = 0 . 39, P = 0 . 008). In a mult iple regression analysis IGFBP-1 was positively correlated with both S HBG and f-testosterone, but not with t-testosterone. CONCLUSION The pr esent study suggests that among healthy young and middle-aged men, the re is an association between serum levels of free-testosterone and SHB G on the one hand, and serum IGF-I and IGFBP-1 on the other.