[Lotus]

Testosterone is responsible for sex drive, for Testosterone to promote youthful sexual interest it must be freely available to cell receptors. As we age T becomes bound to serum (blood) globulin. The component that renders free testosterone inactive is called sex hormone binding globulin (SHBG). This means your getting a double whammy of interference with sex drives in aging males,

1) Testosterone being bound to serum globulin (blood)
2) SHBG rendering it inactive

So that means estrogen takes it place!

Imagine if we knew more about Testosterone, we would find out that a reduction of free T also causes depression. High serum levels of estrogen also trick the brain into thinking that enough testosterone is being produced, further slowing the natural production of testosterone. This also stops the body's own natural estrogen from entering cells for binding.

High estrogen can shut down the normal testicular production of testosterone. Aging men sometimes convert testosterone to estrogen. For testosterone to produce long-lasting libido enhancing effects, it must be kept in the "free" form in the bloodstream. It does not matter how much serum free testosterone is available if excess estrogen is competing for the same cellular receptor sites.