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Testosterone

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Also known as: Total Testosterone; Free Testosterone; Bioavailable Testosterone
Formal name: Testosterone
Related tests: FSH, LH, SHBG, DHEAS, Estradiol (see Estrogens), DHT, Gonadotropin-releasing Hormone, Semen Analysis, Anti-Mullerian Hormone, Androstenedione

The Test Sample

What is being tested?

Testosterone is a steroid hormone (androgen) produced by special endocrine tissue (the Leydig cells) in 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. This test measures the level of testosterone in the blood.

In males, testosterone stimulates 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 in adult males to regulate the sex drive and maintain muscle mass. In women, testosterone is converted to estradiol, the main sex hormone in females.

Testosterone production is stimulated and controlled by luteinizing hormone (LH), which is manufactured by the pituitary gland. Testosterone works within a negative feedback mechanism: as the testosterone level increases, LH production decreases, which slows testosterone production; decreased testosterone causes increased production of LH, which in turn stimulates testosterone production. Testosterone levels are diurnal, peaking in the early morning hours (about 4:00 to 8:00 am), with the lowest levels in the evening (about 4:00 to 8:00 pm). Levels also increase after exercise and also decrease with age. About two-thirds of testosterone circulates in the blood bound to sex-hormone binding globulin (SHBG) and slightly less than one-third bound to albumin. A small percent (about 1-4%) circulates as free testosterone.

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.

How is the sample collected for testing?

A blood sample is taken by needle from a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.