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Doctor with teenager discussing treatment for Klinefelter syndrome.

Doctor with teenager discussing treatment for Klinefelter syndrome.
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Klinefelter syndrome

3-minute read

Klinefelter syndrome is a genetic condition affecting males. It occurs if a man is born with an extra X chromosome. It can cause a variety of problems, including a small penis, small testes and infertility.

What is Klinefelter syndrome?

Klinefelter syndrome is a congenital condition in which males are born with one or more extra X chromosomes. It is not inherited.

Klinefelter syndrome is common, affecting one in every 450 men. But most people with Klinefelter syndrome don’t know they have it and have never received treatment.

It is also known as Klinefelter’s syndrome and as XXY syndrome.

Boys and men with Klinefelter syndrome are unable to produce sperm or enough of the male hormone, testosterone, for the body’s needs. The low levels of testosterone affect the development of male characteristics. The extra X chromosome also affects the ability to produce sperm. Men with this condition are infertile as they almost always have no sperm in their ejaculate (azoospermia).

What causes Klinefelter syndrome?

Most boys and men have one X and one Y sex chromosome. Klinefelter syndrome occurs when a boy is born with one or more extra X chromosomes. It is not clear why this happens.

Klinefelter syndrome symptoms

Most men with Klinefelter syndrome lead the same sort of lives as everybody else. Some people are affected only very mildly, while for others it has more of an impact on their lives. Most babies with Klinefelter syndrome do not have noticeable symptoms.

Young boys with Klinefelter syndrome might have:

Sometimes, issues show up at puberty. If you have Klinefelter syndrome, you didn’t get the rush of the male hormone testosterone that others got. You might have noticed you have:

  • a small penis and small testes
  • less hair on your face and body
  • breasts that are a bit bigger than expected

You might also feel a bit different, which can bring about its own problems. You may also be tall for your age. Other men don’t realise they have Klinefelter syndrome until they find they can’t father children. Some may never find out.

The condition is diagnosed with blood tests.

How can Klinefelter syndrome affect my health?

Most men with Klinefelter syndrome can have a normal sex life but are infertile. If you have Klinefelter syndrome, you are at more risk than other men of getting:

Klinefelter syndrome treatment

Testosterone therapy can help with many of the symptoms of Klinefelter syndrome. This treatment should begin at puberty or as soon after as possible.

Men with Klinefelter syndrome can now have children, if they want, with new forms of in-vitro fertilisation. For example, a single sperm can be injected into a woman's egg (called Intra-cytoplasmic sperm injection). Counselling and emotional therapy may also be helpful.

More information

For more information, you should consult your doctor, endocrinologist or health professional. The following organisations can also provide further information:

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: April 2020

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