Diagnostic Uncertainty in a Patient With Late-Onset Hypogonadism-Like Symptoms: A Case Report.

Late-onset hypogonadism (LOH) is a clinical concept that integrates symptoms with decreased testosterone and affects sexual function, physical vitality, and psychological or behavioral domains. In patients whose complaints traverse multiple specialties, symptom nonspecificity can constrain hypothesis updating. Variation in biochemical assessment, including measurement timing and thresholds, can amplify reliance on treatment response. A 52-year-old Japanese man presented with fatigue, awakening after sleep onset, flushing, palpitations, and erectile dysfunction despite internal medicine evaluations. Vital signs, electrocardiography, blood tests, CT, and MRI suggested no organic disease, and he was referred to urology for suspected LOH. Blood tests showed total testosterone 3.79 ng/mL (not ≤2.5 ng/mL) and free testosterone 7.5 pg/mL (at the threshold). At the patient's request, a trial course of testosterone replacement therapy with testosterone enanthate was initiated, with tadalafil. Sexual function improved, and flushing and edema decreased after several days, followed by recurrence of edema, sweating, and fluctuating fatigue by three weeks. Symptoms persisted, and attribution remained unsettled. Psychiatric consultation was added. This case visualizes how entry-point framing, biochemical borderline results, and symptom overlap can prevent diagnostic convergence. It supports a parallel-evaluation diagnostic design that prespecifies competing hypotheses and interprets treatment response as supportive but non-decisive evidence.