In many cases, measurement of total testosterone provides the doctor with adequate information. However, in certain cases, for example when the level of SHBG is abnormal, a test for free or bioavailable testosterone may be performed as it may more accurately reflect the presence of a medical condition.
Sex Hormone Binding
The sex hormone binding globulin (SHBG) test measures the concentration of SHBG in the blood. SHBG is a protein that is produced by the liver and binds tightly to testosterone, dihydrotestosterone (DHT), and estradiol (an estrogen). In this bound state, it transports them in the blood as an inactive form. The amount of SHBG in circulation is affected by age and sex, by decreased or increased testosterone or estrogen production and can be affected by certain diseases and conditions such as liver disease, hyperthyroidism or hypothyroidism, and obesity. Changes in SHBG levels can affect the amount of testosterone that is available to be used by the body's tissues.
A total testosterone test does not distinguish between bound and unbound testosterone but determines the overall quantity of testosterone. If a person's SHBG level is not normal, then the total testosterone may not be an accurate representation of the amount of testosterone that is available to the person's tissues.
A testosterone test measures the amount of the male hormone, testosterone, in the blood. Both men and women produce this hormone. In males, the testicles produce most of the testosterone in the body. Levels are most often checked to evaluate signs of low testosterone: In boys -- early or late puberty and in men -- impotence, low level of sexual interest, infertility, thinning of the bones
In females, the ovaries produce most of the testosterone and levels are most often checked to evaluate signs of higher testosterone levels, such as: decreased breast size, excess hair growth, increased size of the clitoris. irregular or absent menstrual periods and male-pattern baldness or hair thinning.