Synonyms:  Bioavailable Testosterone; Free and Albumin-Bound Testosterone

Why It Is Done

• Determine whether a problem with the testicles or pituitary gland is preventing a man from being able to father a child (infertility). A low amount of testosterone can lead to low sperm counts.
• Evaluate a man's sexual problems. A low level of testosterone may reduce a man's sex drive or cause impotence (erectile dysfunction).
• Determine whether increased testosterone production is causing a boy younger than age 10 to have early (precocious) puberty.
• Evaluate why a woman is developing male features, such as excessive facial and body hair (hirsutism) and a deep voice.
• Evaluate irregular menstrual periods in women.
• Determine the response to testosterone-lowering medications in men with advanced prostate cancer.
• Evaluate the cause of osteoporosis in a man.

Test Overview

A testosterone test measures the level of this male hormone (androgen) in the blood. Testosterone affects sexual features and development. In men, it is produced in large amounts by the testicles. In both men and women, testosterone is also produced in small amounts by the adrenal glands; and, in women, by the ovaries.
The release of testosterone is controlled by a hormone called luteinizing hormone, or LH, which is produced by the pituitary gland (see an illustration of the pituitary gland  ). When the testosterone level is low, the pituitary gland releases LH, which increases the amount of testosterone produced by the testicles.
Most of the testosterone in the blood is attached to a protein called sex hormone binding globulin (SHBG). A small amount is attached to albumin. The unattached, or "free," testosterone may be measured when conditions that can increase SHBG (such as obesity or hyperthyroidism) are present.

How To Prepare

No special preparation is required before having this test. Your health professional may recommend a morning blood test, when testosterone levels are highest.