SUN-868 - Non-Mosaic Klinefelter Syndrome in A 19-Year-Old Male Detected at A Military Intake Physical.

Klinefelter syndrome (KS) [47,XXY] is a relatively common chromosomal disorder often overlooked as its physical features can be subtle and nonspecific. Recognizing KS early in often asymptomatic adolescents enables early counseling on fertility prospects and subsequent monitoring for possible metabolic and endocrine complications. Our case highlights the unusual settings in which KS can first come to clinical attention and need for vigilance in primary care settings.

A 19-year-old male was referred for evaluation of small testes noted during a routine military intake physical. He reported normal pubertal development, including deepening of voice at age 14, male-pattern hair growth, and normal penile size (7.7 cm / 3.6 inches). He wasnt sexually active and denied trauma, radiation exposure, or use of steroids, opioids, tobacco, or alcohol. Family history was notable for his father having surgically corrected undescended testes.
Physical examination revealed height 190 cm, arm span 160 cm, upper-to-lower segment ratio 0.85, bilateral testicular volumes of 8 cc (left) and 10 cc (right), and symmetric Tanner stage 3 gynecomastia. Mid-parental height was calculated at 177 cm. Laboratory evaluation showed AM total testosterone 308-347 ng/dL on repeat measures .LH (8.0 mIU/mL) and FSH 8.9 (mIU/mL) were normal. While AMH levels were normal Inhibin B was undetectable. Testicular ultrasound was reportedly unremarkable. Karyotype confirmed non-mosaic 47,XXY with no evidence of mosaicism. Semen analysis showed azoospermia.
The patient was counseled regarding KS, including reproductive, endocrine, and psychosocial implications. Testosterone replacement therapy was not indicated and he was referred for formal genetic counseling. He has been cleared for military service. Long-term follow-up will include periodic endocrine, cardiometabolic surveillance, and reproductive health management..

Non-mosaic KS may present subtly with small testes, gynecomastia as the only noteable clinical sequelae. Despite impaired spermatogenesis, patients like ours can have “normal” gonadotrophin and testosterone levels. AMH was normal, whereas inhibin B was low ( < 1 pg/mL), highlighting impaired spermatogenesis despite relatively preserved leydig cell activity. The absence of mosaicism predicts a low likelihood of fertility confirmed by his azospermia . With a prevalence of ~ 1 in 500 to 1000 new born males, KS is a fairly common chromosomal disorder. Consequently, primary care clinicians should consider karyotype evaluation in adolescents or young adults with disproportionate testicular size and/or subtle pubertal abnormalities, even when serum testosterone and gonadotrophin levels are normal. Early identification is critical to enable timely counseling on fertility options, need for long term endocrine and cardiometabolic surveillance, and psychosocial support.

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