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Klinefelter Syndrome

Medically reviewed by: Judith L. Ross, MD

What Is Klinefelter Syndrome?

Klinefelter syndrome can cause problems with learning and sexual development in guys. It's a genetic condition (meaning a person is born with it).

Klinefelter syndrome only affects males. It happens because of a difference deep inside the body's cells, in microscopic strings of DNA (what make up genes) called chromosomes. Chromosomes are tiny, but they play a huge role in who we are — including deciding our gender, how we look, and how we grow.

Doctors sometimes call Klinefelter syndrome "XXY." Instead of one X and one Y chromosome like most guys have, guys with Klinefelter syndrome are born with an extra "X" chromosome in most or all of their cells.

When puberty starts and guys' bodies begin to make sex hormones, boys with Klinefelter usually don't produce as much of the male hormone testosterone. That doesn't make a guy less male, but it can affect things like penis and testicle growth, and growth of body hair and muscles. Boys with Klinefelter syndrome may also have problems with attention, speech development, and learning word skills like spelling, reading, or writing.

Doctors think Klinefelter syndrome is fairly common. Most boys who have it go on to have a good and healthy life, especially if they get the help they need from doctors, speech therapists, and other experts.

What Causes Klinefelter Syndrome?

Klinefelter syndrome isn't passed down through families like some genetic diseases. Instead, it happens randomly from an error in cell division when a parent's reproductive cells are being formed. If one of these cells is part of a successful pregnancy, a baby boy will have the XXY condition.

The XXY condition can affect some or all of the body's cells. If only some cells have the extra X chromosome, it's called mosaic Klinefelter syndrome. Guys with this condition often have milder signs and symptoms. Some may not even know they have Klinefelter syndrome until later in life. Other boys have more than two X chromosomes. Their condition may be more severe.

What Are the Signs & Symptoms of Klinefelter Syndrome?

Differences in boys with Klinefelter syndrome can range from mild to severe. Some guys have few or no obvious signs. Other times, Klinefelter syndrome interferes with speech, learning, and development.

Boys with Klinefelter syndrome might have some or all of these physical signs:

  • a taller, less muscular body than other boys their age
  • broader hips and longer legs and arms
  • larger breasts (gynecomastia)
  • weaker bones
  • a lower energy level
  • smaller penis and testicles
  • delayed puberty or only going through puberty partway (or, in rare cases, not at all)
  • less facial and body hair following puberty

Boys with Klinefelter syndrome may have difficulty with spelling, reading, writing, and paying attention. They may tend to be quiet, shy, and more reserved or sensitive than other guys. Some boys with Klinefelter syndrome are less interested in sports or physical activities.

Since high-school life often revolves around schoolwork and sports, guys with Klinefelter may feel like they don't fit in or lack self-confidence. But, as men, most have normal friendships and relationships.

What Problems Can Happen?

Most teens with Klinefelter syndrome aren't likely to have major health problems. But the condition can bring challenges later in life. For example, guys with Klinefelter syndrome may be more likely to get some types of cancer and other diseases, like type 2 diabetes and osteoporosis, a condition where the bones become weaker later in life.

As men, most guys with Klinefelter syndrome can have sex, usually with the help of testosterone treatment. But problems with their testicles prevent them from making enough normal sperm to father children. Fertility researchers are working on new treatments. By the time a teen with Klinefelter syndrome is ready to become a dad, there may be new options available related to the extraction of sperm from the testicles.

How Is Klinefelter Syndrome Diagnosed?

The earlier a guy finds out about Klinefelter syndrome, the better. Treatments are most effective if they start while a guy is still young. So if you're worried about your development, talk to your doctor. It might be nothing, but it's always best to know, especially because Klinefelter syndrome can be hard to notice.

To diagnose a guy with Klinefelter syndrome, doctors usually begin by asking about any learning or behavior issues and examining the testicles and body proportions. They'll check a blood sample for the presence of the extra X chromosome. Doctors also can do hormone testing, usually by taking a blood sample to check for abnormal hormone levels.

How Is Klinefelter Syndrome Treated?

There's no way to change the XXY condition if someone is born with it, but there are treatments that can help. Beginning treatment early makes it work better.

  • Testosterone replacement therapy (TRT) works by increasing a guy's testosterone levels into the normal range. This can help someone with Klinefelter syndrome develop bigger muscles, a deeper voice, growth of the penis, and facial and body hair. It also can help improve bone density and reduce any breast growth. However, testosterone therapy cannot increase testicle size or prevent or reverse infertility.
  • Physical therapy, occupational therapy, and speech therapy can build muscle strength and coordination as well as help guys improve their speaking, reading, and writing skills.

What Else Should I Know?

It's not easy to feel like you're developing differently from other guys. Guys with Klinefelter syndrome are more likely to have low self-confidence or shyness, which can make things harder. Counselors and therapists can give guys practical skills to help them feel more confident in social settings.

If you're struggling to keep up or do well in school, talk to your school counselor or teachers. Tell them about your condition. You might be able to get extra assistance and support to make schoolwork and classes go more smoothly.

Despite physical differences and other problems, with the right medical care, early intervention, and ongoing support, guys with Klinefelter syndrome can lead a normal, healthy, and productive life.

Medically reviewed by: Judith L. Ross, MD
Date reviewed: September 2021