Testosterone testing is used to diagnose several conditions in men, women, girls, and boys. Testosterone is the main sex hormone in men, produced mainly by the testicles, and is responsible for male physical characteristics. Although it is considered to be a "male" sex hormone, it is present in the blood of both males and females. 

• The testosterone test may be used to help evaluate conditions such as:
• Delayed or precocious (early) puberty in boys
• Decreased sex drive in men and women
• Erectile dysfunction in men
• Infertility in men and women
• Testicular tumors in men
• Hypothalamus or pituitary disorders
• Hirsutism and virilization in girls and women

Typically, a test for total testosterone is used for diagnosis. The total testosterone test measures testosterone that is bound to proteins in the blood (e.g., albumin and sex-hormone binding globulin [SHBG]) as well as testosterone that is not bound (free testosterone).

About two-thirds of testosterone circulates in the blood bound to SHBG and slightly less than one-third bound to albumin. A small percent (less than 4%) circulates as free testosterone. Free testosterone plus the testosterone bound to albumin is the bioavailable testosterone, which can act on target tissues.

In many cases, the total testosterone test provides 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.

In females, testosterone testing may be done when a woman has irregular or no menstrual periods (amenorrhea), is having difficulty getting pregnant, or appears to have masculine features, such as excessive facial and body hair, male pattern baldness, and/or a low voice. Testosterone levels can rise because of tumors that develop in either the ovary or adrenal gland or because of other conditions, such as polycystic ovarian syndrome (PCOS)