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.
However, they also inhibit the bodys own production of sperm and testosterone, which can lead to shrinkage of the testicles.Testicular atrophy is also a side effect of estrogen therapy. COVID-19
COVID-19 can lead to reduced testosterone production and testicular abnormalities due to changes in the secretion of gonadotropins, which are hormones involved in regulating testosterone production. These hormonal effects may be due to inflammation and oxidative stress caused by COVID-19. This might inform whether healthcare providers decide to monitor those who have survived COVID-19 for hormonal changes and potential fertility issues. Orchitis
Orchitis, or inflammation of the testicles due to a bacterial or viral infection, can lead to testicular atrophy. Mumps has historically been significantly associated with orchitis and testicular atrophy, but has become rare in countries that have high mumps vaccination rates. Varicoceles and testicular torsion
Varicoceles and testicular torsion are conditions in which direct damage to the testicles can potentially lead to testicular atrophy. Varicoceles are pooling of blood in the veins that deliver blood away from the testicles and are relatively common, occurring in approximately 15% of adolescent and adults with testicles. Currently, it is not clearly established how often varicoceles actually become a direct cause of issues related to fertility. Testicular torsion is the twisting of the testicle within the scrotum and can rapidly result in damage due to interruption in blood flow to the affected testicle. Unlike varicoceles, testicular torsion is considered a medical emergency. Other health conditions
There is a possible correlation of cystic fibrosis and its impact to bilateral testicular atrophy.