The testicles are oval-shaped male reproductive glands. They sit inside the scrotum, which is a thin membrane of skin that hangs below and behind the penis.

The function of the testicles is to produce sperm and male hormones, such as testosterone.

Most adult males have two testicles, and each typically measures around 4 x 3 x 2 centimeters (cm). However, testicle size can vary. It is also common to have one testicle that is smaller than the other.

Keep reading for more information about testicle size and growth and how to perform a self-examination.

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Testicle size may affect the production of testosterone and sperm.

In general, the size of the testicles does not directly affect health. However, some studies in animals suggest that testicle size may affect the amount of sperm that a male produces.

For example, a 2011 study in sheep found that testicle size directly related to the production of both testosterone and sperm. This study also suggested that larger testicle size may be more attractive to females. However, researchers have yet to show whether these results apply to humans.

While a person with smaller testicles may produce less testosterone, there may also be some advantages. A 2013 study suggested that men with slightly smaller testicles may have a more nurturing parenting style.

The study looked at activity in a part of the brain involved in paternal nurturing behavior. Participants with smaller testicles showed greater activation in this area when viewing pictures of their children, compared with those who had larger testicles.

Hypogonadism

Some males have lower testosterone levels than others. Doctors refer to unusually low testosterone levels as testosterone deficiency or male hypogonadism. Typical symptoms can include:

  • testicles that are smaller than average
  • less facial hair or less male-pattern body hair
  • growth of breast tissue
  • symptoms similar to those of Klinefelter syndrome, which we describe below

Hypogonadism may develop during puberty, and a doctor will usually treat the condition with testosterone replacement therapy.

Klinefelter syndrome

Some males have smaller testicles as a result of Klinefelter syndrome. This results from being born with an extra X chromosome.

Klinefelter syndrome can cause the following symptoms:

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During puberty, the testicles start to grow to their full size.

The testicles start to grow to their full size during puberty. The growth stops when the testicles are fully developed.

In general, both testicles tend to grow at the same rate.

However, it is common for one testicle to be slightly smaller than the other. It is also common for one testicle to hang slightly lower than the other.

Testicle size can fluctuate due to changes in temperature. The size of the testicles may also decrease with age.

Temperature-related changes in testicle size

Changes in temperature can temporarily cause the testicles to draw in closer or descend farther from the body.

For example, the testicles may appear smaller when immersed in colder water. Once the body warms up again, the testicles generally return to their usual size.

This is a natural occurrence — the testicles shrink and expand to keep the sperm at a constant temperature.

Age-related changes in testicle size

As the body ages, the testicles grow smaller. The medical name for this is testicular atrophy (TA).

TA tends to be a gradual process. It may occur so slowly that the person does not notice the change in size.

Other symptoms of TA can include reduced muscle mass and a gradual loss of sex drive.

Some health conditions can cause the testicles to shrink. If signs of these occur, a person should seek medical treatment.

Examples of issues that can cause the testicles to shrink include:

Anyone who has had sex without protection should receive testing for STIs. An STI may cause one or more of the following symptoms:

However, depending on the infection, a person may experience no symptoms at all.

If trauma to the testicles occurs, undergo a medical examination as soon as possible.

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If someone discovers a lump on their testicle, they can talk to their doctor.

Males should examine their testicles at least once a month to check for any abnormalities.

Doctors usually recommend performing these examinations after a hot bath or shower. At this time, the skin of the scrotum is most relaxed, making it easier to feel the testicles within.

The American Urological Association provide the following tips for testicular self-examinations:

  • Perform the test standing up.
  • Look for any swelling in the scrotum.
  • Gently feel the scrotal sac to find one of the testicles.
  • Gently roll the testicle between the thumb and fingers to feel its entire surface.
  • Repeat the process with the other testicle.

Check carefully for any:

A person may feel a cord-like structure, called the epididymis, at the base of each testicle. These bundled tubes carry sperm from the testicles.

Contact a doctor about any changes or lumps in the testicles as soon as possible. The doctor will perform a close examination for signs of disease.

Testicular cancer is one rare but highly treatable disease involving the testicles. If a doctor diagnoses and treats this cancer in its early stages, it is usually curable.

Testicle size does not generally indicate a cause for concern. Most males have one testicle that is smaller than the other, a testicle that hangs lower than the other, or both.

Certain issues and health conditions, such as testicular cancer, can cause changes in the size and shape of the testicles. Performing regular testicular self-examinations can help catch these diseases early and allow for the best chance of successful treatment.