Studies found that after age 30, testosterone levels may decline an average of 2% a year. This decline is a result of several concurrent changes. First is a decline in testosterone production consistent with a decline in leydig cell number in the testes and decreasing activity of the enzymes that produce testosterone. There is also a diminished response to pituitary signals that normally initiate testosterone production and diminished coordination of the release of the pituitary signals that are produced, decreasing any chance for the testes to continue a normal pattern of testosterone secretion. Finally, hormone-binding globulin (SHBG) levels increase with age. These proteins "cling" to testosterone, so even though testosterone may be present, it is not "free" or biologically available to do its work. Increasing SHBG levels, therefore, reduce free testosterone to a greater extent than the reduction seen in total testosterone.