Serum testosterone and bioavailable testosterone correlate with age and body size in hypogonadal men treated with testosterone undecanoate (1000 mg IM--Nebido).

OBJECTIVE: To investigate the loading regimen for intramuscular (IM) testosterone undecanoate (Nebido) to determine whether testosterone and bioavailable testosterone levels achieved correlate with age or body size of subjects studied.

DESIGN: Retrospective observational study of testosterone naïve patients and patients previously treated with an alternative testosterone therapy.

PATIENTS: 51 hypogonadal men (35, 68.6% secondary hypogonadism). 8 (16%) had not previously received testosterone therapy.

MEASUREMENTS: Patients received an IM injection of Nebido (1000 mg) at baseline and a second injection after 6 weeks. Serum was assayed at baseline and 18 weeks after commencing Nebido for total testosterone (TT) and SHBG. Bioavailable testosterone was calculated (cBioT) using TT and SHBG. Measurements were taken for weight, body mass index (BMI) and body surface area (BSA).

RESULTS: Baseline TT (mean 11.5 nmol/l, range 0.3-54.8) increased by 50% after commencing Nebido (17.2 nmol/l (5.4-32.8), P = 0.0001). 75% of subjects had a TT within the reference range (8.0-25.0 nmol/l). Subjects with primary hypogonadism had a higher 18-week TT [20.9 nmol/l (9.8-32.8) vs. 15.5 (5.4-32.6), P = 0.02] and SHBG [39.2 nmol/l (11-82) vs. 25.7 (9.0-60.0), P = 0.003] although the cBioT was not significantly different [4.9 nmol/l (2.9-7.3) vs. 4.2 (2.0-7.9), P = 0.12]. The 18-week TT positively correlated with age (R = 0.36, P = 0.01) and negatively correlated with weight (R = -0.38, P = 0.006), BMI (R = -0.42, P = 0.002) and BSA (R =-0.38, P = 0.007). Similarly cBioT correlated with age (R = 0.28, P = 0.04), weight (R = -0.29, P = 0.03), BMI (R = -0.30, P = 0.03) and BSA (R = -0.27, P = 0.05). Age (t = 2.04, P = 0.05) and baseline testosterone (t = -9.26, P < 0.0001) were independent variables of the increase in TT at 18 weeks.

CONCLUSION: This starting regimen is simple and provides the majority of men with a TT within the reference range. Age and baseline TT are independent variables of the increase in TT with IM testosterone undecanoate. At week 18 age and body size correlated with the cBioT and TT and this may then be used to estimate dosing frequency for this therapy.