Testosterone (Free)

Though often considered a male sex hormone, testosterone is present in females in smaller amounts. In both sexes, testosterone helps control your sex drive and helps with muscle and bone growth.

All the testosterone in your blood is either bound to proteins or unbound (free). Free testosterone is more available for your body to use when building bones and muscles. However, most of your testosterone is bound to proteins such as sex-hormone-binding globulin (SHBG) and albumin. When testosterone is bound, your body can't use it as easily. This is necessary for your health. Having most of your testosterone bound to proteins makes sure your body doesn't use too much.

Only a small fraction of your total testosterone is free, and a free testosterone test focuses on this active portion. A free testosterone result can be especially helpful when total testosterone is near the lower end of normal or when conditions affect SHBG levels. In some cases the free testosterone value is calculated from total testosterone, SHBG, and albumin rather than measured directly.

Your health care provider can measure your testosterone levels by ordering one of several types of testosterone tests.

A testosterone level test alone can't diagnose any health conditions, but abnormal results with symptoms may be a sign of a health problem.

In men, low free testosterone with low energy, low sex drive, or erectile problems may suggest hypogonadism. If LH and FSH are also high, the problem is usually in the testes themselves; if LH and FSH are low or normal, the issue is more often with signals from the brain (the pituitary or hypothalamus).

Free testosterone is especially helpful when total testosterone is borderline or when conditions affect SHBG (the main protein that carries testosterone in the blood). For example, obesity, insulin resistance, and steroid use can lower SHBG, while aging, liver disease, and estrogen use can raise it. In these situations, total testosterone can be misleading even when the active (free) testosterone is normal or abnormal.

In women, mildly raised free testosterone is a common finding in polycystic ovary syndrome (PCOS), where excess male-type hormones cause irregular periods, acne, or extra body hair. Much higher levels can suggest an ovarian or adrenal tumor and need further testing.

If you take testosterone replacement, your provider will check that the level stays in the normal adult range — not so high that it causes side effects, and not so low that symptoms continue. It's normal for testosterone to slowly decrease with age, so an abnormal result doesn't always mean a problem that needs treatment.