Sex hormone binding globulin and testosterone levels

• approximately 60-70% of testosterone is bound to sex hormone binding globulin (SHBG)
  
  
• SHBG levels are lowered via exogenous androgens, obesity and abnormally high growth hormone levels. This reduction in SHBG levels also leads to a reduction in total testosterone concentrations
  
  
• SHBG may be increased via thyrotoxicosis, oestrogens and liver failure. This increase in SHBG levels leads to an increase in total testerone concentrations

Despite changes in the SHBG, free testosterone levels generally remain within the normal range because of feedback adjustment of gonadotrophin secretion. There is an increase in SHBG capacity with age.

• concentrations of sex hormone-binding globulin (SHBG) increase with age
  • age-related decline in serum free or bioavailable (non-SHBG-bound) testosterone is greater than that of total testosterone
    • this change in SHBG concentration with age means that a larger proportion of older men have levels of free or bioavailable testosterone below the normal range for younger men (1)

Reference:

• (1) Gruenewald DA, Matsumoto AM. Testosterone supplementation therapy for older men: potential benefits and risks.J Am Geriatr Soc. 2003 Jan;51(1):101-15
• (2) ) Drug and Therapeutics Bulletin 1999; 37 (1):3-6.