Testicular atrophy is a medical condition in which one or both testicles (or "testes") diminish in size and may be accompanied by reduced testicular function. Testicular atrophy is not related to the temporary shrinkage of the surrounding scrotum, which might occur in response to cold temperature.

As the testicles are involved in testosterone and sperm production, the signs and symptoms of testicular atrophy overlap with those related to infertility or low testosterone levels. In prepubescent testicular atrophy, there may be underdevelopment of secondary sex characteristics (e.g. lack of penis growth). In sexually developed individuals, testicular atrophy may be accompanied with lower sex drive and increased breast tissue. Additional signs and symptoms vary and can depend on the specific cause of the testicle shrinkage. Some causes include age, alcohol use, anabolic steroid use, testosterone replacement therapy, direct damage to the testicles, and infection.

Diagnosis of testicular atrophy includes physical examination of the testicles as well as imaging to measure testicular volume. A testosterone blood level is also taken to assess function of the testicles. Additional tests may be ordered depending on the suspected cause(s) of the shrinkage. Treatment and potential reversibility of testicular atrophy are also dependent on the cause.^{[medical citation needed]}

There are different signs and symptoms associated with testicular atrophy depending on the person's age. Symptoms before puberty are centered more around the stunting of sexual characteristics associated with hormonal changes, while symptoms after puberty include a wider range of factors. Other signs and symptoms may stem from underlying conditions which cause testicular atrophy. The most perceptible sign of testicular atrophy is shrinkage of the testicle(s).

Signs and symptoms before puberty include:

Testosterone is a hormone that is found primarily in a portion of the male reproductive system called the testes and is normally measured in nanograms per deciliter. Testosterone is in charge of the growth and production of many sexual attributes in males, including facial hair, pubic hair, penis size, vocal and muscle mass changes, regulation of sex drive, and sperm production. Normal testosterone levels in males who have not yet hit puberty are less than 20 nanograms per deciliter. Low testosterone can be defined as hypogonadism in people with testes. Hypogonadism in people with testes is established as the hormonal inhibition of testosterone that can either be inherited or acquired at a later stage in life. Under the circumstances of pre-pubescent hypogonadism, many sexual developmental characteristics may be altered. Testosterone affects hair growth by regulating the follicle itself, which in turn affects the specific growth phases of the hair follicle. As a person with testes hits puberty, androgen, a steroid sex hormone, is produced at an increased rate, which creates terminal hair follicles. Terminal hair follicles create thicker and more pigmented hair which is also regulated by testosterone production. Testosterone also impacts pre-pubescent penis size by providing penile tissue with girth and density. The drop in testosterone values is mainly due to significant impairment of Leydig cells brought upon by hypogonadism. Leydig cells are located in the testes and serve to create testosterone and androgen. Once they are damaged, the production of testosterone is stunted.

Signs and symptoms after puberty include:^{[medical citation needed]}

Typical testosterone levels of people with testes after puberty range from 300 to 1,200 nanograms per deciliter. Low testosterone values in males post-puberty would be considered any value under 300 nanograms per deciliter. Decreased testosterone values caused by testicular atrophy can lead to and explain many of the signs and symptoms listed above, including a reduced sex drive, infertility, soft testicles, reduction of hair, and lower muscle mass throughout the body. Testosterone plays an important role in physiologically stimulating sex drive in people with testes. If testosterone levels drop below normal values, libido (sexual drive) is reduced, which can lead to the development of erectile dysfunction (a condition in which the penis is not able to get or stay erect). This can indirectly affect fertility as well. In addition, testosterone is important in muscle mass formation. It increases the amount of lean muscle mass in the body. Testosterone encourages metabolic activity in the body, which in turn stimulates protein synthesis. Protein synthesis is highly important for the development and enlargement of muscle. Therefore, a lack of testosterone would alter this metabolic process, leading to a decrease in muscle mass.