Testosterone is the main sex hormone in males. During a boy's puberty, testosterone causes the growth of body hair, muscle development, and deepening of the voice. In adult men, it controls sex drive, maintains muscle mass, and helps make sperm. Women also have testosterone in their bodies, but in much smaller amounts.

This test measures the levels of testosterone in your blood. Most of the testosterone in the blood is attached to proteins. Testosterone that is not attached to a protein is called free testosterone. There are two main types of testosterone tests:

Total testosterone, which measures both attached and free testosterone.
Free testosterone, which measures just free testosterone. Free testosterone can give more information about certain medical conditions.
Testosterone levels that are too low (low T) or too high (high T) can cause health problems in both men and women.