Testosterone testing is used to diagnose several conditions in men, women, and boys. These conditions include:

• delayed or precocious (early) puberty (boys); 
• decreased sex drive (men);
• erectile dysfunction (men);
• infertility (men and women);
• testicular tumors (men); and 
• hirsutism and masculinization, also called virilization (women).

In boys, the test is ordered, often along with the FSH and LH tests, if puberty is delayed or slow in developing. Although there are differences from individual to individual as to when puberty begins, generally by the age of 10 years, there are hormonal and physical manifestations of the onset of puberty. A delay can occur if the testes do not produce enough testosterone or if the pituitary does not produce enough LH.

The test also can be ordered if a young boy seems to be undergoing a very early (precocious) puberty with obvious secondary sex characteristics, such as an enlarged penis, development of muscle mass, and growth of body hair. Causes of precocious puberty in boys, due to increased testosterone, include various tumors and congenital adrenal hyperplasia.

In men, the test may be ordered when infertility is suspected or if the patient has a decreased sex drive or erectile dysfunction, all of which can result from low testosterone levels.

In women, testosterone testing may be done if a patient has irregular or no menstrual periods, is having difficulty getting pregnant, or appears to have masculine features, such as facial and body hair, male pattern baldness, and 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).

There is great variability in testosterone levels between men and a broad range in age-related values for testosterone. It is normal for testosterone levels to decline as men age.

However, in males, a decreased testosterone level may indicate hypothalamic or pituitary disease or damage to the testes. Genetic diseases also can cause decreased testosterone production in young men (Klinefelter’s, Kallman’s, and Prader-Willi syndromes) or testicular failure and infertility (as in myotonic dystrophy, a form of muscular dystrophy). A decreased testosterone level also can indicate impaired testosterone production because of acquired damage to the testes, such as alcoholism, physical injury, or viral diseases like mumps.

Increased testosterone levels in males can indicate testicular tumors , adrenal tumors that are producing testosterone, or use of androgens (also called anabolic steroids). Increased testosterone in boys is usually the cause of early puberty.

In women, increased testosterone levels can indicate PCOS or an ovarian or adrenal gland tumor.

Alcoholism and liver disease in males can decrease testosterone levels. Drugs, including androgens and steroids, can also decrease testosterone levels. Prostate cancer responds to androgens, so many men with advanced prostate cancer receive drugs that lower testosterone levels.

Women taking estrogen therapy may have increased testosterone levels. Anticonvulsants, barbiturates, and clomiphene can cause testosterone levels to rise.