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(A-Z listing includes diseases, conditions, tests and procedures)

Klinefelter Syndrome

What is Klinefelter syndrome?

Most men are born with a pair of XY chromosomes, while most females are born with pair of XX chromosomes. Klinefelter syndrome occurs when a male is born with an extra X chromosome. Thus a male with Klinefelter has XXY instead of the usual XY pair. Because males with this condition produce less of the male hormone testosterone than other males, they are less masculine-looking than their peers. The condition usually leads to infertility in adult life. Klinefelter has been linked to increased risk for autoimmune disorders, such as lupus and rheumatoid arthritis, as well as depression and learning disabilities. 

Klinefelter occurs in one out of every 500 to 1,000 newborn males.


  • Delayed or incomplete sexual development during puberty 

  • Feminine features 

  • Underdeveloped external genitalia 

  • Sparse or absent facial and body hair 

  • Enlarged breasts 

  • Learning problems 


The definitive diagnosis is karyotype testing that shows the extra X chromosome. 

Other tests may include: 

  • Semen count 

  • Blood tests to measure the levels of certain sex hormones 

When to Call for Help

If a boy fails to mature sexually during puberty, call your health care provider. 


Testosterone therapy can improve many of the signs and symptoms associated with the disorder.


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