Testosterone is a sex hormone. It plays an integral role in developing "male" physical characteristics, as well as sex drive, fertility, and sperm production. Testosterone levels may also impact bone and muscle mass and red blood cell production. Typically, testosterone levels are highest just after puberty and decline with age.
Testosterone is a sex hormone that plays a key role in the development of the male reproductive system and secondary sex characteristics, as well as in male fertility and sex drive. As a person ages, they may naturally produce less testosterone.
Testosterone plays a role in many areas. While it's been coined as the quintessential "male hormone" and is often associated with agression or sex drive, it’s more complex than that. (Plus, assigned females at birth also produce and require testosterone.)
Testosterone is important for many functions throughout life. In utero, it's responsible for the creation and growth of the penis and the testicles. It's also responsible for the development of secondary sex characteristrics during puberty, like muscle growth, body hair, and a deeper voice. Testosterone contributes to bone growth and strength, sex drive, and sperm production throughout adult life.
Having abnormally low levels of testosterone can negatively impact a person's development and fertility. For instance, a young person with low levels of testosterone may not experience normal genital development. Adult cis men with low levels of testosterone may experience low libido, weight gain, fatigue, decrease in muscle mass, moodiness, and depression.
However, supplementing with testosterone is not a solution. Endogenous (or added) testosterone can lead to a decreased production of follicle-stimulating hormone, which is a key hormone for sperm production. This can result in low sperm counts and infertility.
Testosterone isn't the only hormone that drives male fertility. In fact, while testosterone is required for sperm production, it does not need to be in especially high levels.
Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are two other critical male fertility hormones that impact sperm production. FSH is responsible for the function and maturation of the Sertoli cells, which are essential for sperm production. LH is responsible for testosterone production in the testicles.
A few other hormones that impact male fertility include estrogen, prolactin, and thyroid-stimulating hormone. Although these hormones do not directly affect sperm production, they all play a role in fertility.