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Klinefelter Syndrome

Lea este articulo en EspanolLots of boys are slower to develop than their peers, and many boys are quiet and shy. There’s a normal range when it comes to physical and social development. But sometimes these qualities show up alongside learning problems or trouble paying attention in school. When this happens, it could be an indication that something else is going on.

If you’re worried about your son’s emotional or physical development, talk to your doctor. Occasionally, symptoms like these can be linked to a condition called Klinefelter syndrome.

About Klinefelter Syndrome

Klinefelter syndrome, also known as XXY syndrome, is a fairly common genetic condition found in males only. It occurs when a boy is born with an extra sex chromosome in most or all of his cells. Many boys with Klinefelter syndrome have no signs or symptoms, and some don’t even know they have it until later in life.

Klinefelter syndrome typically causes a boy’s testicles to grow at a slower rate than those of other boys. It prevents testicles from producing normal amounts of sperm and the hormone testosterone. Testosterone affects the way a boy develops both physically and sexually. Low hormone levels and problems with sperm production make it difficult or sometimes impossible for a boy with Klinefelter syndrome to father a child later in life.

The XXY condition that causes Klinefelter syndrome can’t be changed, but treatment and working with therapists can aid a boy’s development and help minimize the condition’s effects. Even without treatment, most boys with Klinefelter syndrome grow up to live productive, healthy lives.


Everyone’s genetic material, or DNA, is contained in tiny structures called chromosomes that are found inside their cells. A person’s gender is determined by their sex chromosomes. Females have two X chromosomes, or XX. Most males have one X chromosome and one Y chromosome, or XY. Males with Klinefelter syndrome are born with cells that have an extra X chromosome, or XXY.

Klinefelter syndrome is a genetic condition, meaning people are born with it, but it’s not passed down through families like some genetic diseases. Instead, Klinefelter syndrome occurs randomly due to an error in cell division when a parent’s reproductive cells are being formed. If one of these defective cells contributes to a successful pregnancy, the baby will have the XXY condition in some or all of his cells. Some boys will even have more than two X chromosomes, which increases the risk of severe symptoms and other health concerns.

When a boy is born with the XXY condition in only some of his cells, it’s called mosaic Klinefelter syndrome. Often, boys and men with this condition can have milder signs and symptoms than others with the XXY condition.

Signs & Symptoms

Not all boys with Klinefelter syndrome will have noticeable symptoms. Other boys can have symptoms that are physically apparent or ones that interfere with their speech, learning, and development.

Babies with Klinefelter syndrome typically have weak muscles, reduced strength, and quiet personalities. They also can take longer to do things like sit up, crawl, walk, and speak.

Compared with other kids their age, boys with Klinefelter syndrome might have some or all of the following symptoms:

  • a taller, less muscular body
  • broader hips and longer legs
  • larger breasts (a condition called gynecomastia)
  • weaker bones
  • a lower energy level
  • smaller penis and testicles
  • delayed or incomplete puberty (some boys won’t go through puberty at all)
  • less facial and body hair following puberty

In addition to physical symptoms, many boys with Klinefelter syndrome will also show symptoms related to their development of social and language skills. They may have difficulty paying attention. A lot of boys learn to talk late or have trouble using words to express their emotions. They also can have trouble with things like learning to spell, read, and write.

Socially, boys with Klinefelter syndrome tend to have quiet, docile personalities. They rarely cause trouble and are often more helpful and thoughtful than other boys. They are often shy and sensitive, and many are less self-confident and less active than other boys their age.

Since boys with Klinefelter syndrome can have problems with schoolwork and sports, they may feel like they don’t fit in with other kids their age. But by the time they are men, most will have normal social relationships with friends, family members, and others.

Related Problems

The main complications of Klinefelter syndrome are related to problems with speech, learning, and physical and social development. Most boys aren’t likely to have major health problems, but the condition can bring challenges later in life. Klinefelter syndrome puts males at greater risk of breast cancer, other cancers and diseases, problems with blood vessels, problems with sexual function, and osteoporosis (weak bones) later in life.

Most boys with Klinefelter syndrome can have sex when they become men, usually with the help of testosterone treatment. However, problems with their testicles prevent them from making enough normal sperm to father children.

The vast majority of men with Klinefelter syndrome are infertile and can’t father a child the normal way. At present, options for becoming natural parents are limited, but fertility researchers are constantly working on new treatments. By the time a child with Klinefelter syndrome is ready to become a dad, there may be new options.


Since Klinefelter syndrome can be hard to notice, many parents don’t know their son has it until he grows up or shows delays in puberty. Sometimes, parents who are worried about their son’s development consult a doctor, and the diagnosis reveals Klinefelter syndrome. This can help, because the earlier a boy is diagnosed with Klinefelter syndrome, the more effective the treatments usually are.

To diagnose cases of Klinefelter syndrome, doctors usually begin by asking about any learning or behavior issues and examining a guy’s testicles and body proportions.

There are two main tests used to confirm a diagnosis of Klinefelter syndrome:

  1. Hormone testing, which is usually done by taking a blood sample to check for abnormal hormone levels.
  2. A chromosome analysis, or karyotype analysis, which is usually done on a blood sample. This test checks the number of chromosomes to see if the XXY syndrome is present.


There’s no way to change the XXY condition if a boy is born with it, but treatments can help relieve some of the symptoms. As with many conditions, beginning treatment early can greatly increase its effectiveness.

Testosterone replacement therapy (TRT) works by increasing a boy’s testosterone levels until they are within the normal range. Additional testosterone can help a boy with Klinefelter syndrome develop bigger muscles and a deeper voice, as well as promote growth of the penis and facial and body hair. It can also help improve bone density and reduce the growth of a boy’s breasts. Testosterone therapy cannot increase the size of a boy’s testicles or reverse infertility, however.

Educational support services can help boys and teens with Klinefelter syndrome keep pace in school. Many benefit from extra assistance when it comes to schoolwork. If your son has Klinefelter syndrome, let his teachers and school nurse know about his condition and see what kind of support is available. He may be eligible for an individualized education plan (IEP) or 504 education plan, which both can provide accommodations for kids with special needs.

Speech therapy and physical therapy can help boys with Klinefelter syndrome learn to speak, read, and write better, or improve muscle strength and coordination.

Other forms of therapy include behavioral, mental health, and occupational therapy. These can help improve low self-confidence, shyness, and delayed social development.

Supporting Your Son

It can be difficult for boys with this condition, who feel like they’re developing differently from their peers. They’re more likely to have low self-esteem, which can make things harder socially and academically.

If your son is struggling in school or having trouble making friends, talk to your doctor, school principal, or school counselor. Counselors and therapists can give boys practical skills to help them feel more confident in social settings. And many schools provide supplementary educational services or accommodations that can help your son succeed.

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