Objective: The duration of action of injectable testosterone undecanoate (TU) in routine clinical practice outside clinical registration trials is not well defined.
Design and Methods: Prospective observational study of consecutive TU injections as testosterone replacement therapy for pathological hypogonadism, subject to individual dose titration to optimise achieve a stable replacement regimen. Participants had primary hypogonadism (PH, n=118), secondary hypogonadism (SH, n=85) or were female-to-male transgender (F2M, n=94). After first and 6-week 1000mg loading doses, initially 12-weekly injections were then subject to titration of injection interval based on individual’s lead symptoms and trough serum testosterone, LH and FSH.
Results: Among 6300 injections given to 297 patients having at least three injections (median number of injections 14, interquartile range [IQR] 7-25), the mean inter-injection interval was 11.9 ± 0.02 (SEM) weeks (mode and median 12 weeks, IQR 11.0 – 12.4 weeks). The optimal stabilised injection interval was significantly influenced by age, body surface area (BSA) and serum SHBG but not diagnosis. Shorter (≤10 weeks) intervals in 23/297 (7.7%) patients were positively correlated with age and longer (≥14 weeks) intervals in 37/297 (12.5%) patients were positively correlated with age and BSA. Serum LH was fully suppressed in the PH group to levels comparable with SH, whereas in F2M serum LH was minimally suppressed (vs pre-titration) and significantly less than PH and SH groups despite higher dose/BSA. Serum FSH was markedly but not fully suppressed in men with PH while in F2M serum FSH was not suppressed.
Conclusion: After individual dose titration to optimise interval between TU injections, the approved 12-week duration of action was observed in 80% and influenced by age, serum SHBG and BSA but not diagnosis. Shorter and longer intervals in 20% of patients depended mainly on age and BSA.
Supported by the Genetic Hypogonadism Registry