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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.