What is Klinefelter syndrome?

Klinefelter syndrome (KS) is a genetic condition in which boys are born with an extra copy of the X chromosome (piece of genetic material). A person with KS has 47 chromosomes inside their cells instead of the usual 46. KS is also called 47, XXY.

Males with KS typically have small testicles that make less testosterone (male sex hormone) than usual. Many men also have low sperm counts that can lead to infertility.

Other common challenges in males with KS include learning disabilities and trouble with social interactions.

What causes Klinefelter syndrome?

Klinefelter syndrome happens at random. It is not anyone’s fault. KS is not passed down from parent to child.

What are the symptoms of Klinefelter syndrome?

KS symptoms are different for every person. In most cases, symptoms do not show up until the teen or young adult years. In other cases, symptoms show up during infancy or childhood.


  • Low muscle tone
  • Speech delays
  • Undescended testicles (when one or both testicles have not dropped into their usual position outside of the body)


  • Learning difficulties
  • Attention deficits (ADHD)
  • Behavioral concerns

Teens and young adults

  • Tall stature
  • Small testes
  • Gynecomastia (breast enlargement)
  • Anxiety and depression


  • Fetility challenges
  • Low sex drive
  • Metabolic syndrome (high cholesterol, high blood pressure, diabetes)

How do doctors diagnose Klinefelter syndrome?

Symptoms of KS are often subtle. They can also look similar to other medical conditions. This can make KS more challenging for doctors to diagnose.

In many cases, doctors diagnose KS during or after puberty. Tests can include:

  • A physical examination
  • Genetic testing
  • Hormone testing (done through a blood sample)

KS can also be diagnosed prenatally (before birth). Tests can include:

  • Cell-free DNA testing
  • Chorionic villus sampling
  • Amniocentesis

How do doctors treat Klinefelter syndrome?

There is no cure for KS. Treatment focuses on you or your child’s individual symptoms and medical needs. Your care team will talk with you to create an individualized treatment plan. This will often include being seen by different types of doctors including a clinical geneticist, an endocrinologist or a fertility specialist, and a developmental medicine specialist or a psychologist.

Treatment options for KS can include the following:

  • Testosterone replacement therapy to help replace the testosterone the body does not make. You can meet with an endocrinologist (hormone doctor) to decide if this option is appropriate. Testosterone replacement therapy comes as a gel, patch or injection.
  • Surgery to remove extra breast tissue, if present
  • If you are an adult or your child is a teen who wants children in the future, fertility treatments can help improve the chances of having children.


  • Speech and language therapy to help children cope with speech delays or challenges
  • Physical therapy to build muscle strength
  • Occupational therapy to gain everyday skills for living and work
  • Behavioral therapy or counseling to cope with depression, anxiety or low self-esteem
  • Help in school to cope with learning challenges or disabilities

Rev. 1/2020. MassGeneral Hospital for Children and Massachusetts General Hospital do not endorse any of the brands listed on this handout. This handout is intended to provide health information so that you can be better informed. It is not a substitute for medical advice and should not be used to treatment of any medical conditions.