SHBG stands for Sex Hormone Binding Globulin.  In some literature this compound is also referred to as Testosterone Binding Globulin, or TeBG.

Circulating testosterone is present in several forms. Testosterone may be present as a free hormone (not bound to any protein) or bound relatively weakly to albumin. The majority of testosterone in circulation, however, is bound to sex hormone binding globulin (SHBG). The testosterone bound to SHBG is not available for biological activity. The free testosterone and that weakly bound to albumin comprise the so called "bioavailable" testosterone fraction which is responsible for peripheral androgenic effects. Which is the most important measurement in diagnosing hypogonadism, the total T, or free T remains controversial.

SHBG binds with testosterone, estrogen, and other sex hormones, rendering them unavailable for reactive purposes.  By contrast, albumin binds only weakly to testosterone. Testosterone which is albumin-bound will dissociate rather easily, at which point it becomes available for reacting with a receptor site or conversion into dihydrotestosterone.  Generally, about half of a man's total testosterone is SHBG-bound, about three to five percent is free, and the remainder is albumin-bound.

Albumin can be thought of as a transport mechanism for testosterone; it will bind testosterone sufficiently that the bound combination won't react. Thus bound, the agglomeration travels through the bloodstream to peripheral tissues. At some point the testosterone dissociates and promptly reacts wherever it may be at that time. Without a protective transport mechanism, the testosterone likely would not make it out to peripheral tissue in normal concentration; testosterone is just too highly reactive for that.

-  Thanks to Steve Gerken

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