[Lotus]

What are free and bioavailable testosterone?

Testosterone is present in the blood as "free" testosterone (1-4%) or bound testosterone (~98%). The latter may be loosely bound to albumin (38%), the main protein in the fluid portion of the blood, or bound to a specific binding protein called Sex Hormone Binding Globulin (SHBG) (60%), also called Sex Steroid Binding Globulin (SSBG). The binding between testosterone and albumin is not very strong and is easily reversed, so the term bioavailable testosterone (BAT) refers to the sum of free testosterone plus albumin-bound testosterone.

It is suggested that bioavailable testosterone represents the fraction of circulating testosterone that readily enters cells and better reflects the bioactivity of testosterone than does the simple measurement of serum total testosterone. Also, varying levels of SHBG can result in inaccurate measurements of bioavailable testosterone. Decreased SHBG levels can be seen in obesity, hypothyroidism, androgen use, and nephritic syndrome. Increased levels are seen in cirrhosis, hyperthyroidism, and estrogen use. In these situations, measurement of free testosterone may be more useful.

Women's bodies also produce testosterone but in small amounts. It is needed for hormonal balance and to help women's bodies to function normally. If your body is producing too much testosterone, you may have more body hair than average, have abnormal or no menstrual periods, or be infertile. A testosterone test, in conjunction with measuring other hormone levels, can help your doctor to understand what is causing your symptoms.

http://labtestsonline.org/understanding/...ab/faq#top


So what then controls your free testosterone? It turns out that your level of albumin is relatively fixed with minimal fluctuation in most men. However, SHBG can vary widely and lab results clearly show that as SHBG goes up, the percentage of free testosterone as a percentage of total goes down and, as SHBG goes down, the percentage of free testosterone goes up. Thus, it is SHBG that has tremendous control over free testosterone.

Normal aging, as many of us know all too painfully, leads to substantial drops in androgens such as testosterone and a corresponding rise in SHBG. Think how ugly this is: decreasing testosterone leads to increasing SHBG which leads to decreased free testosterone. And, practically speaking, low free testosterone levels will yield low testosterone symptoms just as surely as low total testosterone will.

There are also many other reasons for low and high SHBG, including liver dysfunction, estradiol levels, thyroid function and much more. I cover these in my links on Low SHBG and High SHBG.

NOTE:  Men on HRT (Hormone Replacement Therarpy or testosterone therapy) will often have an elevated free testosterone percentage, because HRT tends to lower SHBG. Also, some men have low SHBG even before starting HRT. These men often have great difficulty with achieving successful results from testosterone therapy for reasons that are not totally understood.
http://www.peaktestosterone.com/Free_Tes..._SHBG.aspx