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What Causes Low Testosterone?

You probably know the important role testosterone plays in a man’s life. It’s the stuff that makes a man a man, from facial hair and a deeper voice to sex drive, erections, and sperm production. When a man has low testosterone, his libido can plummet, along with other aspects of his sexual function.

So you might wonder what causes low testosterone. And is there anything you can do to maintain your testosterone levels, even as you age?

Let’s define what we mean by low testosterone. Testosterone is measured with a simple blood test. Most healthy adult men have testosterone levels between 270 and 1,070 nanograms per deciliter (ng/dL). 300 ng/dL is usually the threshold for a low testosterone diagnosis.

But keep in mind that a man’s testosterone levels fluctuate during the day. Levels are usually highest around 8 a.m. and lowest around 9 p.m. Most doctors conduct testosterone tests early in the morning so they can get a consistent reading over time.

Another thing to think about is the way testosterone is produced. Most of it is made in the testes, but before that even happens, signals from the pituitary gland and the hypothalamus (a part of the brain) need to trigger that production. The pituitary gland and hypothalamus are just as important as the testes.

Now, let’s look at some of the reasons behind low testosterone.


Aging. For most men, testosterone levels start decreasing around age 40 and continue to decrease about 1% each year. So by age 70, your levels can decline by about 30%. The good news is that even with the drop, three-quarters of older men still have testosterone levels in the normal range.

Obesity. Some of a man’s testosterone is naturally converted to estrogen, a hormone usually associated with women. But men need estrogen, too, especially to maintain healthy bone density. The problem with obesity is that the conversion from testosterone to estrogen mainly happens in fat cells. The more fat cells you have, the more testosterone is being converted to estrogen, leading to lower testosterone levels.

Injury to the testicles or scrotum. Injured testes are sometimes unable to produce the amount of testosterone a man needs. Interestingly, amounts can remain stable if only one testicle is injured. The healthy one can still produce enough testosterone on its own.

Chemotherapy and radiation therapy. These therapies can damage cells in the testes that make testosterone. Sometimes, levels return to normal if the cells recover, but sometimes the damage is permanent.

Medications. Opiates, taken for pain, and certain hormones can cause problems with testosterone production.

Performance enhancing drugs (anabolic steroids). Bodybuilders and athletes sometimes take anabolic steroids to make them stronger or faster. But performance enhancing drugs can make testicles shrink and impair testosterone production. They are also illegal, when used in this way.

Inflammation. Certain conditions and diseases, such as sarcoidosis, histiocytosis, tuberculosis, and HIV/AIDS can affect the pituitary gland and/or the hypothalamus because of inflammation.

Infection. Mumps, meningitis, and syphilis are known to lower testosterone levels.

Head trauma and tumors. These conditions can also affect the pituitary gland and hypothalamus.

Too much iron in the blood (hemochromatosis). This can cause damage to your testes and your pituitary gland.


Is there anything I can do to keep my testosterone levels from decreasing?

Maybe. Keeping yourself fit and healthy – important for so many reasons – is also important for your testosterone. Protect your testicles when you play sports. Make sure you get enough exercise, including resistance exercises and strength training. Eat a healthy diet full of fruits and vegetables and high-fiber foods. Watch your fat intake. Practice safe sex and don’t abuse drugs and alcohol.

Taking these steps can help prevent some of the causes of low testosterone, such as obesity, cancer, and HIV/AIDS. Plus, you’ll improve your overall health and your sex life. It’s a win-win situation.

If I have low testosterone, should I have hormone replacement therapy (HRT)?

Talk to your doctor. There are pros and cons to hormone replacement therapy. Researchers are still unsure how much HRT helps a man’s sexual function overall and there are other factors that can affect your sex life as much as testosterone.