Mediastinal Tumor in a Boy With GnRH-Independent Precocious Puberty and Fluctuating β-HCG Levels

Gonadotropin-releasing hormone (GnRH(-independent premature puberty in boys, characterized by elevated β-human chorionic gonadotropin (β-hCG) levels, can indicate a secreting germ cell tumor (GCT). These tumors are rare but more common in individuals with Klinefelter syndrome (KS). We present a case of a 7.3-year-old boy with precocious puberty. Physical examination revealed bilateral testicular volumes of 8 to 10 mL and Tanner stage 3 secondary sexual characteristics (genitalia G3, pubic hair P3). His skeletal age was 12 years. Biochemical tests showed suppressed gonadotropin levels, elevated testosterone, and increased β-hCG of 86.6 mIU/mL (86.6 IU/L, reference range: <5 mIU/mL, <5 IU/L). Imaging, including magnetic resonance imaging (MRI), chest x-ray, whole-body computed tomography (CT), and testicular ultrasound, were interpreted as normal except for a small pineal cyst. Karyotype testing confirmed KS. Over 10 months, β-hCG levels fluctuated between 1 to 105 mIU/mL (1-105 IU/L). When β-hCG was 3.6 mIU/mL (3.6 IU/L), a fluorodeoxyglucose positron emission tomography-CT (FDG PET-CT) scan revealed a mediastinal tumor. The tumor was surgically removed and identified as a mature teratoma. This case underscores the importance of karyotype testing and repeated imaging in boys with premature puberty and elevated β-hCG levels, even if β-hCG levels decrease spontaneously and remain low.