A number of things affect both SHBG as well as testosterone. Age, exercise, obesity just to name a few. However, insulin also affects SHBG levels. Hyperinsulinemia is associated with a reduction in SHBG concentration and SHBG concentration is positively associated with insulin sensitivity. SHBG concentration is also negatively correlated with insulin resistance, insulin levels and glucose concentration. Both insulin and insulin-like growth factor 1 have inhibitory effects on SHBG secretion by Hep G2 cells in vitro. Insulin also suppresses hepatic SHBG synthesis.
Because of this relationship, many papers are studying whether SHBG levels can be predictive of the development of diabetes. Because SHBG binds to its receptor and actually has some actions through g-protein/cAMP pathway, there are studies also looking at whether low levels of SHBG may actually lead to diabetes. If this pans out, perhaps SHBG may be a target in the future for something else we have to raise.
1. Stellato RK, Feldman HA, Hamdy O, Horton ES, McKinlay JB. Testosterone, sex hormone-binding globulin, and the development of type 2 diabetes in middle-aged men: prospective results from the Massachusetts male aging study. Diabetes Care. 2000;23(4):490-494.