Though testosterone often gets a bad rap because aggressive or misbehaving men are sometimes labeled as "testosterone fueled," the hormone actually isn't unique to one type of person or gender and affects just about everyone in one way or another. 

Its formation is initiated during gestation, and "normal circulating levels of testosterone" are produced in the body when a person reaches the age of puberty, between 10 and 14 years old, explains Andrew Greenberg, MD, director of the obesity and metabolism lab at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. 

What is testosterone?

Testosterone is a sex hormone that is essential to the development of male growth, per Harvard Medical School. In men, the hormone is produced mainly in the testes, but testosterone isn't exclusive to males alone. "It is also secreted by the ovaries and adrenal glands of women," says Cynthia Stuenkel, MD, a clinical professor of medicine at UC San Diego School of Medicine. However, she adds that the blood level of testosterone in young women "is about 1/10th that in men." 

What does testosterone do?

Testosterone has multiple important functions in the male body including sperm production, deepening of the voice, facial and body hair growth, libido, erectile function "and development of male genitalia," says Disha Narang, MD, an endocrinologist at Northwestern Medicine Lake Forest Hospital. 

Greenberg says the hormone also helps increase bone density and muscle mass, and may "reduce the accumulation of fat in the abdomen which is associated with metabolic disorders."

In women, testosterone plays a role in ovarian function, bone health and sexual health, though "data is mixed" on the latter point, explains Narang.

What does low testosterone do to a man?

Because testosterone levels can vary by individual and usually decline with age, which is "a normal occurrence," says Narang, many people will experience the effects of low testosterone levels, also called low T levels, throughout their lifetime. 

Beyond in people getting older, other individuals that may experience low T levels include people dealing with Type 2 diabetes, obesity, sleep apnea or chronic medical conditions such as kidney dysfunction. 

Symptoms of low testosterone levels may include decreased bone density, lower self-esteem, diminished energy levels, less body hair and a decreased sex drive. And while low T levels have sometimes been associated with erectile dysfunction, research from the Cleveland Clinic shows there is likely not a direct correlation between the two.  

How do you fix low testosterone?

For individuals experiencing symptoms associated with low testosterone levels, Greenberg advises consulting with one's physician. Such providers can take various symptoms into consideration when diagnosing conditions and have many treatment options available to them including things like skin patches, gels, injections and more.

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