The most perceptible sign of testicular atrophy is shrinkage of the testicle(s).Signs and symptoms before puberty include:

lack of development of pubic hair
lack of development of facial hair
lack of penis growth
lower testosteroneTestosterone is a hormone that is found primarily in a portion of the male-assigned 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 people with testes, including facial hair, pubic hair, penis size, vocal and muscle mass changes, regulation of sex drive, and sperm production. Normal testosterone levels in people with testes 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:

reduced sex drive
tender testicles
trouble conceiving
reduction of pubic or facial hair
decreased muscle mass
lower testosteroneTypical testosterone levels of people with testes after puberty range from 300 to 1,200 nanograms per deciliter. Low testosterone values in people with testes 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.