Sex Hormone Binding Globulin - SHBG
SHBG (sex hormone binding globulin) binds to testosterone and estradiol. High SHBG may decrease free testosterone and free estradiol.
What is the SHBG Test?
Testosterone and estradiol circulate in the bloodstream, bound mostly to SHBG (sex hormone-binding globulin) and to some degree other blood proteins. Only a small fraction of the sex hormones are unbound, or "free," and thus biologically active and able to enter a cell and activate its receptor.
SHBG decreases the active form of the hormone available to the tissues. Thus bioavailability of sex hormones is influenced by the level of SHBG. High levels of insulin and androgens decrease SHBG level. On the other hand, thyroid hormone and estrogen increase it.
When our body produces testosterone or provides it exogenously, there is a set level of testosterone flowing through the body. This testosterone blood level can be measured precisely through a simple blood test aptly referred to as the Total Testosterone blood test. For most adult men, when testing their total testosterone, if it falls in the 350 ng/dl to 1100 ng/dl range, this will be considered medically optimal. Such individuals, even if exhibiting symptoms of low testosterone, will often not be prescribed TRT. In some cases, this may be a correct diagnosis; in other cases, symptoms exist for different reasons, but in most cases, something else is at play, which makes SHBG important.
Measuring total testosterone, while useful, is rather useless without also measuring Free Testosterone. Free testosterone is a small fraction of your total testosterone. It is often referred to as "unbound," meaning the testosterone your body will use for its intended androgenic function. Free testosterone is so important that if your total testosterone levels are within range, if free testosterone levels are low, in most cases, the man will be symptomatic of low testosterone. This issue is often due to high levels of SHBG in the body.
For this reason, many men will attempt to lower their SHBG or even fret over it. However, SHBG serves an essential purpose. As is with estrogen, too much can be problematic, but so can too little. SHBG is no different.
Of all the testosterone flowing through your body, only 2% is what we can refer to as unbound or free testosterone. From a layman's perspective, one might think to increase free and unbound testosterone, all they'd need to do is increase their total testosterone. Unfortunately, it doesn't always work that way – it often helps and often fixes the issue but not always – and here's why: our testosterone is bound to two things, albumin and sex hormone-binding globulin (SHBG). Approximately one-third of total testosterone binds to albumin and the rest to SHBG.
Albumin is a protein that while binds testosterone, and we can say it does so loosely. This protein carries the testosterone through the bloodstream and releases it as it's needed. The testosterone detaches, and it becomes what's often referred to as bioavailable or free testosterone. The testosterone that separates from albumin becomes the testosterone that influences your body's functions from sex organs, muscle tissue, brain function, and a host of other areas of bodily function. Testosterone attached to SHBG cannot do this; it is attached and does not detach.
What Causes Low or High SHBG?
Conditions with low SHBG include ovarian cysts, diabetes, and hypothyroidism. Conditions with high SHBG include pregnancy, hyperthyroidism, and anorexia. Some medications like anabolic steroids and high dose testosterone can reduce SHBG.
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