While falling testosterone levels are a normal part of aging, certain conditions can hasten the decline. Risk factors include:

- injury or infection
- chemotherapy or radiation treatment for cancer
- medications, especially hormones used to treat prostate cancer and corticosteroid drugs
- chronic illness
- stress
- alcoholism
- obesity

Symptoms of low testosterone in men include:

- Fatigue, depression, or trouble concentrating
- Hair loss
- Loss of muscle mass
- Low sex drive
- Swollen breasts
- Trouble getting or keeping an erection
- Weak bones -- called osteoporosis

Some things that cause low testosterone can also cause a low sperm count. If your sperm count is low, your doctor might order this test.

Millions of men use testosterone replacement therapy to restore low levels and feel more alert, energetic, mentally sharp, and sexually functional. But it's not that simple. A man's general health also affects his testosterone levels. For instance, being overweight, having diabetes or thyroid problems, and taking certain medications, such as glucocorticoids and other steroids, can affect levels. Therefore, simply having low testosterone levels does not always call for taking extra testosterone.

Doctors diagnose low testosterone based on a physical exam, a review of symptoms, and the results of multiple blood tests since levels can fluctuate daily.

If your doctor diagnoses low testosterone, other tests may be considered before therapy. For example, low testosterone can speed bone loss, so your doctor may recommend a bone density test to see whether you also need treatment for osteoporosis.

Prostate cancer is another concern, as testosterone can fuel its growth. As a result, the Endocrine Society recommends against testosterone supplementation for men in certain situations, including those who:

- have prostate or breast cancer

- have an elevated blood level of prostate-specific antigen (a blood test used to screen for prostate cancer)

- have a prostate nodule that can be felt during a rectal exam.

Other circumstances in which testosterone supplementation is not recommended include:

- a plan to become a father in the near-term

- an elevated red blood cell count

- severe, untreated sleep apnea

- severe lower urinary tract symptoms

- poorly controlled heart failure

- heart attack or stroke within the last 6 months

- a tendency to form blood clots (a condition called thrombophilia)

In most cases, men need to have both low levels of testosterone in their blood and several symptoms of low testosterone to go on therapy. It is possible to have low levels and not experience symptoms. But if you do not have any key symptoms, especially fatigue and sexual dysfunction, which are the most common, it is not recommended you go on the therapy given the uncertainty about long-term safety.

Even if your levels are low and you have symptoms, low testosterone therapy is not always the first course of action. If your doctor can identify the source for declining levels—for instance, weight gain or a particular medication—he or she may first address that problem.

Most of the testosterone in your blood attaches to 2 proteins: albumin and sex hormone binding globulin (SHBG). Some testosterone is free, which means it is not attached to proteins. Free testosterone and albumin-bound testosterone are also referred to as bioavailable testosterone. This is the testosterone that is easily used by your body.

If your healthcare provider thinks that you have low or high testosterone, your total testosterone levels will be checked first. This looks at all 3 types of testosterone. The free testosterone can help give more information when total testosterone is low.

Too much testosterone in men can cause the following:

- Acne
- Increased risk of prostate cancer
- Male breast enlargement (gynecomastia) because some of the excess testosterone gets converted to estrogen
- Sleep apnea
- Aggressive moods
- Elevated blood pressure (hypertension)
- High cholesterol (dyslipidemia)  
- Increased red blood cell count (erythrocytosis) and clotting problems
- Shrinking testicles
- Other causes of high testosterone in men include testicular or adrenal gland tumors.