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Some will show few or no symptoms, a lanky, youthful build and facial appearance, or a rounded body type. By adulthood, individuals with KS tend to become taller than average, with proportionally longer arms and legs, less-muscular bodies, more belly fat, wider hips, and narrower shoulders. As babies and children, those with XXY chromosomes may have lower muscle tone and reduced strength. Generally, the severity of the malformations is proportional to the number of X chromosomes present in the karyotype. In other cases, symptoms are more prominent and may include weaker muscles, greater height, poor motor coordination, less body hair, gynecomastia (breast growth), and low libido. An older mother may have a slightly increased risk of a child with KS. The second X chromosome comes from the father and mother nearly equally.
For data regarding neurocognitive function in individuals with KS, refer to the dedicated review presented in this same journal issue covering this topic. Although the additional X chromosome has global implications, one of the consequences of KS is testicular dysfunction resulting in hypergonadotropic hypogonadism. Taking care of boys and men with Klinefelter syndrome (KS; 47,XXY) presents itself with a number of challenges. We stress the implementation of centralized multidisciplinary clinics and the need for a dedicated international guideline to ensure optimal care of boys and men with KS. \"Gender-affirming care is just not for transgender youth. This will affect cisgender children like Theodore,\" she says.
In the short video, she explained that while he \"looks like a normal toddler,\" he has a chromosomal disorder. \"He is the youngest of three boys. He loves to play with his big brothers, play outside and play in the water. He is so adorable, cuddly and so sweet,\" she tells PEOPLE. Theodore is her youngest son, and Nicklow shares that, aside from his diagnosis, he is a \"typical 2-year-old toddler.\" The mom of three says the only thing Theodore\'s diagnosis has changed about her parenting is that she has now learned to \"slow down and embrace the chaos.\"
The risk of fractures and osteoporosis is increased in KS (Bojesen, Kristensen, et al., 2006), as is mortality due to fractures (Swerdlow, Higgins, Schoemaker, Wright, & Jacobs, 2005). Among 19 men considered to be receiving testosterone at the time of thrombosis, only two presented with hematocrit above normal range. Similarly, Foresta et al. (2012) saw no difference in blood pressure comparing 46 men with KS and normal testosterone levels with 46 men with KS and low testosterone levels. The pediatric oxandrolone trial did not report a change in systolic or diastolic blood pressure with treatment (Davis et al., 2017).
This includes established testosterone replacement therapies, assisted reproductive technologies, and a promising pipeline of targeted hormonal and genetic interventions. However, it is estimated that only 25% of the individuals with Klinefelter syndrome are diagnosed throughout their lives. A 2024 study showed 19% of KS respondents identified as intersex or non-binary, 12% as female and 53% as male, with 56% overall noting some discrepancy between their gender identity and their physical appearance. In August 2022, a team of scientists published a study of a skeleton found in Bragança, north-eastern Portugal, of a man who died around 1000 AD and was discovered by their investigations to have a 47,XXY karyotype. Jacobs described her discovery of this first reported human or mammalian chromosome aneuploidy in her 1981 William Allan Memorial Award address.
The impact of testosterone treatment on quality of life in patients with KS has not been investigated sufficiently. While additional studies with large samples sizes are needed, there is no clear evidence suggesting that testosterone treatment should be withheld in hypogonadal individuals due to the risk of thrombosis. Available studies have mainly focused on the risk of myocardial infarction in the setting of testosterone supplementation, but registry data from UK have also indicated a potential increased risk of venous thromboembolisms 3–6 months after initiation of testosterone treatment in hypogonadal men (Martinez et al., 2016). Most find no effect of testosterone treatment on levels of total cholesterol or LDL cholesterol while two cross-sectional studies report higher triglycerides in treated KS (Chang et al., 2015; Jorgensen et al., 2015) and four cross-sectional studies reporting decreased HDL in treated KS (Aksglaede et al., 2011; Chang et al., 2015; Jorgensen et al., 2015; Zitzmann et al., 2015).
The syndrome is defined by the presence of at least one extra X chromosome in addition to a Y chromosome, yielding a total of 47 or more chromosomes rather than the usual 46. Subjects affected by the condition are almost always phenotypically male (except for SRY-negative individuals), with common complications such as infertility and small, poorly functioning testicles (if present). Klinefelter syndrome (KS), also known as 47,XXY, is a chromosome anomaly.