Klinefelter syndrome is a fairly common genetic
condition found in males only.
Many boys with Klinefelter syndrome — also known as XXY syndrome
— have no signs or symptoms, and some don't even know they have it until later
The XXY condition that causes Klinefelter syndrome can't be changed, but medical
treatment and working with therapists can help a boy's development and help lessen
the condition's effects. Even without treatment, most boys with Klinefelter syndrome
grow up to live productive, healthy lives.
What Causes Klinefelter Syndrome?
Boys who have Klinefelter syndrome are born with it. It's also called XXY because
they have an extra X chromosome in most or all of their cells.
Usually, a person has 46 chromosomes in each cell, divided into 23 pairs, which
includes two sex chromosomes. Half of the chromosomes are inherited from the father
and the other half from the mother. The chromosomes contain genes, which determine
an individual's characteristics, such as eye color and height. Boys typically have
one X chromosome and one Y chromosome, or XY, but boys with XXY syndrome have an extra
X chromosome, or XXY.
Klinefelter syndrome is not caused by anything the parents did or did not do. The
disorder is a random error in cell division that happens 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 (XXXY or XXXXY, for example), 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.
What Are the Signs & Symptoms of Klinefelter Syndrome?
Not all boys with Klinefelter syndrome will have noticeable symptoms. Other boys
can have symptoms that are physically apparent or problems with speech, learning,
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 these symptoms:
a taller, less muscular body
broader hips and longer legs and arms
larger breasts (a condition called gynecomastia)
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
What Problems Can Happen?
Klinefelter syndrome typically causes a boy's testicles to grow at a slower rate
than those of other boys. It also prevents the 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.
Many boys with Klinefelter syndrome show symptoms related to their development
of social and language skills. They may have trouble 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, they tend to have quiet personalities. They rarely cause trouble and
are often more helpful and thoughtful than other boys. They're often shy and sensitive,
and many are less self-confident and less active than other boys their age.
Most boys aren't likely to have major health problems, but the condition can bring
some other challenges later in life. Klinefelter syndrome puts males at greater risk
of breast cancer, some other cancers,
and some other diseases like type 2 diabetes, varicose veins and 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 usual way. Options for becoming natural parents are limited, but fertility
researchers are working on new treatments. By the time a child 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?
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 the boy's testicles and body proportions.
There are two main types of tests used in the diagnosis of Klinefelter syndrome:
Hormone testing, which is usually done by taking a blood sample to check for abnormal
A chromosome analysis, or karyotype, which is usually done on a blood sample.
This test checks the number of chromosomes to see if the XXY syndrome is present.
How Is Klinefelter Syndrome Treated?
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 into 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 prevent 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.
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
Because 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. They're more likely
to have low self-esteem, which can make things harder socially and academically. But
by the time they're men, most will have normal social relationships with friends,
family members, and others.
If your son struggles in school or has 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
educational services or accommodations that can help your son succeed.