(458) Is There an Individual Testosterone “Well-Being Threshold”? Optimal Injection Interval for Testosterone Undecanoate in the Treatment of Hypogonadal Men

Introduction Testosterone undecanoate (TU) is widely used for long-term testosterone replacement therapy (TRT) in hypogonadal men. Despite standardized injection schedules, interindividual variability in symptom recurrence and in serum total (TT) and free testosterone (fT) levels often necessitates individualized adjustments. Identifying a personal testosterone “well-being threshold” for symptom recurrence may optimize dosing intervals and improve therapeutic outcomes. Objective To determine whether each hypogonadal man exhibits a reproducible individual testosterone threshold for the recurrence of symptoms during long-term treatment with intramuscular TU. Methods A cohort of 266 men diagnosed with hypogonadism received TU injections over a period of six years. Serum T, sex hormone-binding globulin (SHBG), and albumin were measured at regular intervals. When patients reported the recurrence of hypogonadal symptoms before the next scheduled injection, blood samples were collected to determine T and calculate fT using Vermeulen’s established equilibrium equations. The fT and TT concentrations at the time of symptom recurrence were compared intra- and interindividually to assess reproducibility and variability of the threshold levels. Results At the time of symptom recurrence, both TT and fT concentrations were highly reproducible within individuals, indicating a consistent personal threshold for hypogonadal symptoms. However, testosterone levels at this threshold varied substantially between individuals, with a range of approximately 9.4 nmol/L for total T. This finding demonstrates that while each man maintains a stable physiological “symptom threshold,” there is marked interindividual variability in the absolute serum concentration associated with symptomatic relief or recurrence. Conclusions Each hypogonadal man appears to possess a distinct, reproducible testosterone threshold at which symptoms of deficiency recur - an individualized “well-being level.” These thresholds differ considerably between individuals, suggesting that fixed injection intervals for TU are suboptimal. Instead, personalized adjustment of dosing intervals based on symptom monitoring and testosterone measurements may enhance therapeutic efficacy, symptom control, and patient satisfaction. Disclosure No