What is Free Testosterone?


Free testosterone is a term that refers to the amount of testosterone in the bloodstream, usually tested in males with certain medical conditions. According to the American Association for Clinical Chemistry (AACC), free testosterone only accounts for 1-4 percent of the testosterone in a typical male’s bloodstream.

Testing of free testosterone is often done for a variety of medical reasons, usually (but not always) in men. Examining what free testosterone is, as well as its function and significance, is essential in understanding the role it plays in overall health.

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What is Testosterone?

Testosterone is one of the hormones that makes men, men. Testosterone affects many functions of the male body, one of most noticeable being sex drive.*

The “male hormone”

Testosterone Molecule

Testosterone is a male sex hormone (androgen) that is principally produced by the testes. Testosterone is an important component the normal development of male sexual organs and prostate. Elevated levels of testosterone during puberty cause the development of secondary sexual characteristics (broadening of shoulders, beard development, etc.). Testosterone plays an important role in the growth and development of bone and muscle, and in the maintenance of muscle strength.

It is important to note that although testosterone is an androgen, it is also manufactured (secreted) and found in much lower levels in females. Elevated levels of testosterone in females may lead to the development of male-like secondary sexual characteristics (masculinization).

Teststosterone and other steroid hormones (e.g., estrogen, cortisone, etc) are derived from cholesterol. At the molecular level, steroid hormones such as testosterone consist of four interlocking rings of carbon atoms (three of the rings contain six carbon atoms and the fourth ring contains five carbon atoms) that with hydrogen atoms bonded to the ringed carbon atoms, form a hydrogenated cyclopentophenanthrene-ring. Testosterone is produced in the interstitial cells of Leydig that lie between the seminiferous tubules.

After secretion by the testes, testosterone enters the blood stream to circulate throughout the body. As the testosterone circulates, it binds to specific receptors on target tissues. Within the cell, testosterone is converted into dihydrotestosterone–and it is in this form that testosterone enters the cellular metabolism. Dihydrotestosterone binds with other proteins to form a complex that is capable of entering the nucleus and stimulating DNA transcription.

Testosterone that fails to bind to tissues degrades into inactive substances that are removed by excretion. Most of these biochemical breakdowns occur in the liver.

Although there are other male sex hormones produced by the testes, testosterone is by far the most abundant hormone produced. Elevated testosterone levels occur at two very different and distinct times in normal male development. Increased testosterone secretion occurs in newborn male infants and with the onset of puberty.

Luteinizing hormone (LH) produced by the pituitary gland stimulates testosterone production in the pubescent and adult male. LH acts in coordination with follicle stimulating hormone (FSH) to promote the production of sperm cells (spermatogenesis). In the adult, increased testosterone secretion causes enlargement of the penis, scrotum, and testes and induce the development of secondary sexual characteristics, including the development of body hair. Interestingly, testosterone inhibits the growth of hair on the top of the head and–along with a genetic predisposition to baldness–may eventually cause baldness. Testosterone also causes a thickening of skin and of the laryngeal mucosa and larynx that produce the deeper male voice. In addition to promoting a higher metabolic rate, testosterone cases increase nitrogen retention and protein synthesis associated with muscle growth.

In addition, testosterone acts to promote calcium retention and bone matrix development that produces the thickened male skeletal structure.

Source: Testosterone from World of Anatomy and Physiology.
©2005-2006 Thomson Gale, a part of the Thomson Corporation.

The importance of testosterone

Testosterone is a critical component of many male secondary sex characteristics which include sex drive, muscle growth, body and facial hair, bone strength and potentially increased rates of cellular metabolism. If someone is experiencing low levels of testosterone, these attributes can be negatively affected, resulting in loss of muscle strength, increased belly fat and loss of sexual desire. To find out more about the effects of testosterone on the body, you can click on any of the following links:

Low Testosterone and Your Health
How Low Testosterone Affects Health, Mood, and Sex
Low Testosterone and Sex Drive
Low Testosterone Explained: How Do You Know When Levels Are Too Low?
Testosterone May Drop When Men Become Dads
Sleep Loss May Lower Testosterone

How to test for testosterone

A testosterone test measures the amount of testosterone in the blood.
There is no test for testosterone levels that can be conducted “at home” but there are laboratories that sell collection kits which you can submit for lab analysis. The best option to get your testosterone levels checked is to go to your doctor. Make sure to ask for both overall testosterone as well as free testosterone levels.

Read more on Testosterone Tests on Medline Plus