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BULGING MUSCLES, a voracious sex drive, and super strength are probably what come to mind when you think about testosterone. And it does play a role in those things. But testosterone does a lot more for the body, and not everyone knows a lot about what testosterone really is.

Testosterone is a hormone that’s mainly produced in the testicles, and it’s the main sex hormone for people assigned male at birth.

“It’s involved in many natural processes in the body and helps maintain muscle mass, bone mineral density, sperm production, red blood cell production, and sex drive,” says Darshan Patel, M.D., assistant professor of urology at the University of California San Diego’s Men’s Health Center.

Normal testosterone levels vary, depending on your age, size, overall health, and even the time of day. Blood or saliva tests can measure your testosterone, and doctors recommend testosterone replacement therapy if your levels are too low, Dr. Patel says.

If your testosterone is too high or too low, you’ll experience a range of symptoms, like erectile dysfunction, low energy, feeling down, acne, and irritability.

Some men whose testosterone is perfectly normal mistakenly think more testosterone is better and will take extra testosterone to increase their levels, says Ryan Smith, M.D., associate urology professor and urologic microsurgeon specializing in men’s health at the University of Virginia Health. “Many men want to drive levels higher and higher, thinking that that's going to make them feel better, and generally, that's not the case,” Dr. Smith says.

Here’s what you should know about testosterone’s main functions and what can affect your T levels.

What’s the Main Function of Testosterone for Men?

Testosterone is a vital hormone that performs many functions for men throughout their lives, including during fetal development, puberty, and adulthood. According to Harvard Medical School, testosterone plays an important role in:

  • The development of the penis and testes
  • Voice deepening during puberty
  • Facial and pubic hair development in puberty
  • Strength and muscle growth
  • Bone growth and strength
  • Sex drive
  • Sperm production
  • The production of new red blood cells

Women also produce testosterone in the ovaries, but levels of the hormone are higher in men. Most of the testosterone that women produce is converted to the female sex hormone estradiol, according to the Cleveland Clinic.

What Testosterone Doesn't Do

The hormone is crucial for men to develop characteristically male traits, like facial hair and muscle development, but higher levels aren’t always better, Dr. Smith says. Some men with normal testosterone levels might use (and abuse) anabolic steroids, which are essentially synthetic testosterone, to increase their levels or fitness performance. Anabolic steroid abuse brings side effects, like reduced sperm counts, baldness, and an increased risk for prostate cancer, according to the Cleveland Clinic.

While testosterone plays a role in sperm production, supplementing with testosterone actually decreases sperm production and can impact fertility, Dr. Patel says. So, it’s important to talk to your doctor before using testosterone therapies.

Testosterone also doesn’t cure erectile dysfunction, explains Dr. Patel. Low testosterone may be a contributor to ED, but for most men, erectile dysfunction is related to changes in how the body responds, delivers, and maintains adequate blood to the penis.

Can Testosterone Levels Be Too High—Or Too Low?

It’s natural for your testosterone level to fluctuate throughout the day. It usually peaks in the morning and falls throughout the day.

Doctors usually test your T levels in the morning and do a second test to ensure someone’s levels are normal for them, Dr. Patel says.

It’s possible for your testosterone to be too low or too high—each end of the spectrum brings its own set of symptoms.

Signs Your Testosterone Is Too Low

There are several symptoms that are directly linked to testosterone deficiency, according to the American Urological Association, including:

  • Lower sex drive
  • Erectile dysfunction
  • Loss of body and facial hair
  • Loss of muscle mass
  • Fatigue
  • Obesity
  • Depressed mood

Some people might also have lower energy, memory problems, and trouble focusing.

Signs Your Testosterone Is Too High

It’s rare for men’s testosterone to get too high naturally. But your levels can rise if you have a testicular tumor (and, of course, if you take large doses of anabolic steroids). According to Harvard, signs your testosterone is too high include:

  • Low sperm counts
  • Acne
  • Weight gain
  • Fluid retention
  • Mood swings
  • Aggressive behavior
  • Muscle mass increases
  • Headaches
  • Enlarged prostate
  • Insomnia

Unnaturally high testosterone might increase your risk for a heart attack, liver disease, and high blood pressure and cholesterol.

What Affects Testosterone Levels?

Many factors can affect your testosterone levels and cause a deficiency, Dr. Patel says. These include:

  • Aging
  • Obesity
  • Poor sleep
  • Sedentary lifestyle
  • Thyroid problems
  • Diabetes
  • Medications, like opioids, steroids, or antidepressants
  • Chemotherapy
  • Conditions, such as HIV/AIDS
  • Damage to (or removal of) the testicles
  • Undescended testicles

High testosterone can be caused by taking steroids or using testosterone replacement therapy without a doctor’s supervision. Testicular or adrenal tumors can cause excess testosterone production.

When to See a Doctor About Your Testosterone Levels

Any time you’re worried about your sex drive or your ability to get an erection, or you just feel tired or down all the time (or you have any of the symptoms listed above), you should talk to your primary care doctor.

It might feel embarrassing to talk about these problems, but treatment is available to help. Your doctor will usually refer you to a urologist for testing.

“This may initially involve testing your testosterone levels (at least twice) and if indicated, a discussion about the risks and benefits of testosterone therapy, along with other conservative therapies,” Dr. Patel says.

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Erica Sweeney
Erica Sweeney is a writer who mostly covers health, wellness and careers. She has written for The New York Times, HuffPost, Teen Vogue, Parade, Money, Business Insider and many more.