Picture of Claus Højbjerg Gravholt
Claus Højbjerg Gravholt
Ph.d., dr.med, Aarhus University

What is Klinefelter syndrome?

Klinefelter syndrome is a condition affecting boys and men born with an extra X chromosome. Chromosomes are present in all cells in the body and contain DNA. Some of the DNA is arranged in genes, which are sequences of DNA that encodes instructions to make molecules called proteins that the body needs to function. Normally people have 23 pair of chromosomes, so 46 chromosomes in total. One of the chromosome pairs is called sex chromosomes. A woman will have two X chromosomes and her karyotype (chromosomal make-up) will then be 46, XX. A man will have the same number of chromosomes, but the sex chromosomes are made up of a X and a Y chromosome and so a man’s karyotype is 46, XY. A man with Klinefelter syndrome usually have an extra X chromosome and the karyotype will then be 47, XXY. 

Common conditions in Klinefelter syndrome

What causes Klinefelter syndrome?

As far as we know, Klinefelter syndrome occurs at random and is not passed on from a parent who has it to a child. The extra X chromosome can be from the mother or the father and occurs with about the same frequency. Klinefelter syndrome is the most frequent sex chromosome abnormality and is seen in about 150 boys out of every 100,000 live-born boys.

Signs and symptoms seen in Klinefelter syndrome

Signs and symptoms vary greatly. Some men see themselves as being without any signs related to Klinefelter syndrome, but a physician would be able to find discrete signs related to the syndrome. Most are not diagnosed before adulthood and the average age at diagnosis is 27 years. At least 50-60% are never diagnosed.

A man with klinefelter syndrome being comforted

Signs and symptoms in different age groups

Newborn and young boys can present with:

Adolescents can also have:

Adults can present with:

  • Cryptorchidism

•       Small testicles

•       Low level of  testosterone

•       Small penis

•       Enlarged breasts (Gynaecomastia)

•       Infertility

•       Weak musculature

•       Long legs and short upper body

•       Reduced libido

•       Speech and language problems

•       Taller than most

•       Erection problems

•       Learning difficulties

•       Reduced muscle mass

•       Neurocognitive problems related to decision             making and learning difficulties

•       Reading difficulties

•       Delayed puberty

•       Overweight

•       Lacking social competences

•       Reduced beard and secondary body hair


•       Behavioural difficulties

•       Low level of energy

A doctor talking to a worried man with Klinefelter syndrome

How is the diagnosis made?

The diagnosis is based on clinical examination, the existence of social and learning difficulties, blood tests for hormones, and a chromosome analysis. The diagnosis can also be made prenatally (while the baby is still in the womb).

How is a male with Klinefelter syndrome treated?

Treatment can alleviate many of the most frequent physical, social and learning difficulties. Boys and men with Klinefelter syndrome should ideally be looked after by a team of specialists. Such a team could consist of a pediatric endocrinologist, an adult endocrinologist, a speech specialist, a clinical geneticist, a psychologist, a plastic surgeon and others. Several different conditions are seen more frequently in Klinefelter syndrome, of which many can be alleviated or avoided by correct information and life-style changes.

It is recommended to treat all males with Klinefelter syndrome when testosterone production becomes deficient. Life-long treatment with testosterone should ensure proper muscle mass, libido, bone mass and the avoidance of being overweight. In addition, it may also improve social function and improve learning abilities.

If you want to read a success story regarding fertility treatment with Klinefelter´s syndrome, then follow the link to Raj´s story about his life with Klinefelter´s syndrome.

Infertility and Klinefelter syndrome

Until recently it was thought that all males with Klinefelter syndrome were infertile. Recent developments have uncovered that many males retain small areas with normal semen production in their testicles. This can be explored with testicular sperm extraction (TESE) often performed with a microscope (micro-TESE). Here, the testicles are opened and the fertility specialist aims to uncover such areas.

A anxious man thinking about klinefelter and infertility
A man consult his doctor regarding Klinefelter syndrome


We know a lot about Klinefelter syndrome today, but there are also a lot of issues that still need further study. For example, there is an urgent need to become better at diagnosing the syndrome early. We also need more information concerning when and how to start treatment with testosterone and the effects that this has on many aspects of life. We also need better intervention tools to improve the frequent neurocognitive challenges that a man with Klinefelter syndrome will face.

In this article, you can read a personal story about life with Klinefelter syndrome