What's Klinefelter syndrome?
Klinefelter syndrome is a sex-chromosome aneuploidy in males with one or more extra X chromosomes (classically 47,XXY). It is the most common cause of primary hypogonadism, affecting roughly 1-2 in 1000 men, and is frequently undiagnosed. Care is supportive, centered on testosterone replacement for the hypogonadism, fertility options where relevant, and monitoring for metabolic, bone, and thrombotic complications.
| Also indexed as | MONDO:0006823 |
|---|---|
| Features mapped | 13 |
| Treatments mapped | 3 |
| Published sources | 10 |
| Last reviewed | 2026-08-04 |
Signs and symptoms
Hypergonadotropic hypogonadism
In Klinefelter syndrome the testes make too little testosterone. The pituitary gland responds by raising the hormones that signal the testes, so luteinizing hormone (LH) runs high while testosterone stays low. This pattern of low testosterone with raised pituitary signals is called primary, or hypergonadotropic, hypogonadism.
Small testes
Small testes are one of the typical signs of the testosterone shortfall in Klinefelter syndrome, alongside reduced sex drive and fewer spontaneous erections.
Azoospermia
Most men have no sperm in the ejaculate (azoospermia) and are infertile by natural means. Sperm can sometimes be retrieved surgically (micro-TESE) for assisted reproduction.
Tall stature
Boys with the condition tend to grow taller than expected for their family, often with relatively long legs.
Gynecomastia
Some develop breast tissue (gynecomastia), particularly in adolescence. Testosterone treatment can reduce it and sometimes avoids the need for surgery.
Language impairment
Language and speech difficulties are common in boys with Klinefelter syndrome; around two-thirds need speech and language therapy or other developmental support, and starting that support early matters.
Venous thrombosis
Blood clots in the veins (venous thromboembolism) happen more often in Klinefelter syndrome. In this cohort the rate was about four times higher than in the general adult male population.
Metabolic dysfunction
Men with Klinefelter syndrome develop metabolic conditions more often than other men. In one large registry study the rate of metabolic conditions, including type 2 diabetes, was more than twice as high.
Infertility
Infertility is the most common reason Klinefelter syndrome is diagnosed in adulthood. The extra X chromosome impairs the testes' ability to make sperm, so most men have very few or no sperm in the semen, though some can father children with assisted techniques.
Reduced libido
Reduced sex drive is one of the common signs of the low-testosterone state seen in Klinefelter syndrome, together with fewer spontaneous erections and small testes.
Learning and cognitive difficulties
Klinefelter syndrome is commonly associated with learning and cognitive difficulties, especially with language, reading, and attention. Intelligence is usually in the normal range, but targeted educational support is often helpful.
Type II diabetes mellitus
Men with Klinefelter syndrome have a more than two-fold higher rate of type 2 diabetes, a condition in which blood sugar runs too high.
Breast carcinoma
Klinefelter syndrome is associated with a higher risk of breast cancer than is seen in most other men.
How it is diagnosed
Klinefelter syndrome
Diagnosed using: karyotype (47,XXY).
“Klinefelter syndrome (KS) is a chromosomal karyotype abnormality, most commonly presenting with the 47,XXY…”
Klinefelter syndrome
Diagnosed using: serum FSH and LH measurement.
“Patients diagnosed with hypogonadism should have serum FSH and LH concentrations measured to distinguish primary from secondary hypogonadism.”
Treatment and management
What the research describes, not a recommendation. Treatment decisions belong with your clinician.
This covers treatments that appear in the published research mapped here. Investigational and experimental therapies are not included, so their absence is a boundary of this map, not a sign they do not exist.
testosterone replacement therapy
Testosterone replacement therapy is the mainstay treatment, used to correct the hormone deficiency. It supports muscle, bone, energy, mood, and sexual function and can reduce gynecomastia, but it does not restore fertility. It is supportive treatment, not a cure for the chromosomal condition.
Used to help with: Klinefelter syndrome.
“34 were on testosterone replacement therapy…”
testosterone replacement therapy
Testosterone replacement therapy is commonly used in Klinefelter syndrome. It is given to address the low-testosterone state and can support well-being, body composition, and sexual function.
Used to help with: Klinefelter syndrome.
“Testosterone replacement therapies are commonly used in Klinefelter syndrome to improve well-being, body composition and sexual function.”
microdissection testicular sperm extraction with ICSI
Although most men with Klinefelter syndrome have no sperm in their semen, sperm can sometimes be recovered directly from the testis using microdissection testicular sperm extraction (micro-TESE). Retrieved sperm can then be used with intracytoplasmic sperm injection (ICSI), and clinical pregnancies have followed this approach.
Used to help with: Klinefelter syndrome.
“Successful Sperm Retrieval and Clinical Pregnancies Following Micro-TESE and ICSI…”
What changes how it shows up
Carrying the genetic change is not the whole story. The factors below are described in the research mapped here as changing whether, or how strongly, the condition appears. They modulate how the genotype is expressed; they do not, on their own, cause or cure it.
mosaicism
Some people have a mosaic form (47,XXY/46,XY), where only a proportion of cells carry the extra X. Mosaic forms tend to be milder and more variable, and can sometimes preserve some fertility.
Described as modulating: Klinefelter syndrome.
“…a mosaic karyotype of…”
47,XXY
The classical form of Klinefelter syndrome carries a 47,XXY karyotype: an extra X chromosome added to the usual male set. It is the most common sex chromosome difference in men.
Described as modulating: Klinefelter syndrome.
“Klinefelter syndrome (KS) is caused by the presence of a supernumerary X chromosome (conferring the classical 47,XXY karyotype) and is the most common sex chromosome abnormality in men.”
How to read the evidence labels
Where this comes from
This guide is built from 10 published source(s). Every claim above links back to one of them. Click any source ID to read the original on PubMed.
Take it further
Printed, source-linked documents built from this condition's graph — ready to bring to an appointment or attach to a coverage request. Every claim carries its published source, the same as this guide.