The influence of age on bioavailable and free testosterone is independent of body mass index and glucose levels

Abstract

To evaluate the influence of age on serum levels of total testosterone (TT), bioavailable testosterone (BT), free testosterone (FT), and sex-hormone binding globulin (SHBG), considering the presence of fasting blood glucose levels and body mass index (BMI) in a selected male population. A total of 428 men were analyzed. Anthropometry was taken from all, considering BMI as general obesity indicative variable. Fasting blood samples were drawn for determination of plasma glucose levels and serum levels of albumin, TT, and SHBG. The values of BT and FT were calculated from TT, SHBG, and albumin, by Vermeulen's equation. Statistical significance was set at P≤0.05. Age was negatively correlated to BT (r=-0.301; P<0.001) and FT (r=-0.273; P<0.001), but not to TT levels (r=0.002, P=0.974). Age was positively correlated to SHBG (r=0.376; P<0.001). Age was independently associated with the occurrence of high SHBG levels (OR=1.07, 95%CI=1.05-1.10, P<0.001) and of low BT (OR=1.04, 95%CI=1.02-1.07, P<0.001) and FT levels (OR=1.05, 95%CI=1.03-1.08, P<0.001), but not with low levels of TT (P=0.08). Age was significantly associated to high levels of SHBG and to low levels of BT and FT, without significant association to TT. This pattern was independent of BMI and glucose levels.