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Genetic, Chromosomal & Metabolic Conditions

Klinefelter Syndrome

Lee este articuloAidan watched as his friends all gradually got deeper voices and body hair. He kept waiting for his turn, but his body didn’t really change much. Aidan’s doctor took a sample of his blood for testing. The test showed that Aidan had a condition called Klinefelter syndrome.

What Is It?

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

Top Things to Know About Klinefelter Syndrome

Klinefelter syndrome only affects males. It happens because of a difference deep inside the body’s cells, in microscopic centers 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 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. 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 It?

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.


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
  • 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 so much of high-school life 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.

Fertility and Other Problems

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 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 constantly working on new treatments. By the time a teen with Klinefelter syndrome is ready to become a dad, there may be new options.

What Doctors Do

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 his testicles and body proportions. If the signs point toward the condition, doctors will order one of two types of tests:

  1. Hormone testing. This test checks for abnormal hormone levels that can result from the XXY condition. It is usually done with a blood sample.
  2. Chromosome analysis. This test is also called a karyotype analysis. It checks for the XXY condition by looking at the number of chromosomes. The test is usually done using a blood sample.


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 more effective.

  • Testosterone replacement therapy (TRT) works by increasing a guy’s testosterone levels until they are within 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 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.

It’s not easy to feel like you are 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.

Reviewed by: Steven Dowshen, MD
Date reviewed: February 2013