Total testosterone, free testosterone, bioavailable testosterone – you may be confused. Which testosterone level is most important?
If you been to this website before you know that we emphasize the fact that testosterone is a total body hormone, not just a muscle or sex hormone. And, that testosterone levels decline as we age (but not necessarily because we age as a recent study suggests that the decline is lifestyle related) creating a variety of signs and symptoms in both men and women.
Men make about 7 mg of testosterone a day (range of 3 mg to 10 mg) with 95% being produced in the testes and the remaining 5% by the adrenal glands. Women also make testosterone (ovaries, adrenal glands, and conversion from other hormones), but a woman’s production is one-eighth to one-tenth that of men. But, women can suffer from low T (low testosterone), too.
Testosterone Level and Binding Proteins
Most of the testosterone we make is bound to 2 proteins; sex-hormone binding globulin (SHBG) and albumin. In fact, 98% of testosterone is bound to these 2 proteins. The remaining 2% or so is called the free testosterone. It is the free testosterone that actually enters the cell, hooks up a testosterone receptor, and then turns that cell on to do what that cell has been programmed to do when stimulated by testosterone (muscle cell will build muscle, a bone cell will build bone, etc.).
The total testosterone is just that – it’s all the testosterone floating through your blood stream and includes the free testosterone, the testosterone bound to albumin, and the testosterone bound to SHBG. It is the most common testosterone level ordered by physicians, but clinically the least useful level, yet the level upon which most treatment guidelines are based upon.
Here’s why the total testosterone may not be the best testosterone level to obtain. A man (or woman) can have a “normal” total testosterone level based on reference ranges, but he or she may have an excessive amount of testosterone bound to protein, thus having a low amount of free testosterone to enter the cell potentially developing symptoms related to low testosterone.
SHBG’s and Albumin’s Affinity to Bind Testosterone
Sex-hormone binding globulin binds to testosterone much more strongly than albumin does to testosterone. SHBG’s affinity to bind testosterone is somewhere in the magnitude of 100 to 1000 times stronger than albumin’s affinity to bind testosterone. What’s the significance of this? First, about 60% to 70% of the total testosterone is bound to SHBG and the remaining 30% to 40% to albumin (keep in mind the 2% that is free). Testosterone can disassociate (become unbound) from albumin if needed, and therefore, is potentially bioavailable for the cells to use. Testosterone bound to SHBG is essentially “locked up” and is not available for use.
You can think of bioavailable testosterone as a reserve that the body can tap into when the free testosterone is running low. So it’s a bit like indicator coming on in your car telling you that you have so many gallons of gas left in your tank. How much bioavailable testosterone ever actually enters the cell and is used in these situations is unknown.
Bioavailable testosterone is calculated from a formula. To apply the formula you need to know your total testosterone, SHBG, and albumin levels.
Now that you understand testosterone physiology better, you’ve probably have already concluded there are 2 ways to increase your free testosterone. The most obvious is to increase total testosterone, and the other way to raise the free testosterone is by reducing the amount of testosterone bound to SHBG.
What Factors Affect Sex-Hormone Binding Globulin?
Sex-hormone binding globulin is affected by many factors. SHBG naturally increases with age. Other hormones affect SHBG and include insulin, growth hormone, testosterone, estrogen, and thyroid. Obesity and insulin resistance lowers SHBG which may sound like a good thing, but many other factors offset that and testosterone levels tend to be lower in diabetics and obese individuals. There is some evidence that SHBG can be affected by herbs and supplements. Green tea is reported to raise SHBG. Vitamin D, boron, and stinging nettle root are reported to lower it.
We get a total and free testosterone on all our patients at every lab draw in addition to other hormones and labs. At this point we believe the combination of both gives us an accurate assessment. We believe the free testosterone level is the most clinically useful level to obtain in our patients. As we learn more about SHBG and the factors that affect it that might be modifiable there may be situations in certain patients where obtaining a SHBG level could be clinically useful (ie. stop drinking so much green tea because it’s raising your SHBG and lowering your free testosterone).
Unfortunately, most physicians simply obtain a total testosterone on their patients. If your doctor does this, ask him or her politely why, and suggest that a free testosterone or bioavailable testosterone level be obtained, too.
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