What is Free Testosterone?
Testosterone is the main sex hormone (androgen) in men. Although it is considered a “male” sex hormone, it is present in the blood of both men and women. However, testosterone levels are normally much higher in men than in women. This test measures the level of testosterone in your blood. It is measured as total testosterone, either alone or in combination with free testosterone and/or bioavailable testosterone.
Testosterone is mainly produced by the male testicles. It is also produced by the adrenal glands in both males and females and, in small amounts, by the ovaries in females.
- In males, testosterone stimulates the development of secondary sex characteristics, including enlargement of the penis, growth of body hair, muscle development, and a deepening voice. It is present in large amounts in males during puberty and adulthood to regulate the sex drive and maintain muscle mass.
- In women, most testosterone is converted to estradiol, the main sex hormone in females.
Testosterone production is stimulated and controlled by luteinizing hormone (LH), which is made by the pituitary gland. Testosterone works within a negative feedback loop:
- A low level of testosterone causes increased production of LH, which in turn stimulates testosterone production.
- As the testosterone level increases, LH production decreases, which slows testosterone production.
Testosterone levels are diurnal, with the highest (peak) levels occurring in the early morning hours (about 4:00 to 8:00 am) and the lowest levels occurring in the evening (about 4:00 to 8:00 pm). Levels also increase after exercise and decrease with age.
About two-thirds of testosterone circulates in your blood bound to the protein sex hormone-binding globulin (SHBG) and slightly less than one-third is bound to the protein albumin, the most abundant protein in the blood. A small percent (less than 4%) circulates as free (not bound to protein) testosterone. Free testosterone plus the testosterone that is bound to albumin is the testosterone that can act on target tissues. This is called bioavailable testosterone.
In many cases, the measurement of total testosterone provides a healthcare practitioner 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.
Why Get Tested?
To detect an abnormal testosterone level in males and females:
- In males, to help diagnose the cause of symptoms, such as erectile dysfunction or the inability of your partner to get pregnant (infertility)
- In females, to help diagnose the cause of masculine physical features (virilization), excess body hair (hirsutism), infertility, or polycystic ovary syndrome (PCOS)
- In children, to help determine the cause of delayed or early (precocious) puberty or the cause of genitals that are not clearly male or female (ambiguous genitalia)
To monitor testosterone levels in transgender individuals who are undergoing hormone therapy
When To Get Tested?
- For males, when you may be infertile or are unable to get or maintain an erection
- For females, when your voice is lower in quality or you have excess body hair, when you have abnormal uterine bleeding, do not menstruate (amenorrhea), or cannot get pregnant (infertility)
- When a child has signs of delayed or early puberty or has genitals that are not clearly male or female
A blood sample was drawn from a vein
Test Preparation Needed?
None, but the sample is typically collected in the morning.