Endocrine Abstracts

Background: Testosterone (T) in a hydroalcoholic gel is commonly prescribed as androgen replacement therapy and may not be accompanied by peaks and troughs of serum T levels as seen in T-injectables. We assessed in 17 subjects the differences in serum testosterone (sT) and free T (fT) levels during four different application periods.

Methods: Patients where on daily 50 mg Androgel^®, for 7 consecutive days in each cycle, on day 8 serum was sampled from both antecubital veins at 9.00 pm. Period 1 started with application at both shoulders and right upper arm up to 10 cm from the elbow pit. Period 2 was equivalent to period 1 except that T-gel was applied to the left arm. Period 3 was similar to the second except T-gel was applied up to the left elbow pit. Period 4, T-gel was solely applied on the abdomen.

Results: No significant differences in (free) T levels were observed between regimens 1 and 2. However, in period 3 sT and fT levels (median±S.D.) sampled from the left arm were higher compared to the right arm (sT left 15.60±22.17 nmol/l, right 12.20±4.70 nmol/l, P=0.011; fT left 0.39±0.74 nmol/l, right 0.29±0.15 nmol/l P=0.012). Significant lower sT (7.35±3.45 nmol/l, P<0.05) and fT (0.16±0.08 nmol/l, P<0.05) were observed when T-gel was applied to the abdomen compared to all other applications.

Conclusion: Peaks of sT concentrations are observed during T-gel therapy with different application regimens. Serum sampling within the T application area may reveal high T levels, possibly due to local T-accumulation. This should be taken into account with interpretation of sT levels and dose titration during transdermal androgen substitution. Efficacy of T application at the shoulders may be higher compared to application at the abdomen.