Context: The optimal frequency for on-treatment serum T measurement used for dose adjustment after transdermal T gel application is unknown, especially in older men with thinner skin and slower metabolic clearance.
Objectives: The objectives of the study was to determine the variability of postgel application serum T concentrations and assess whether single levels are reflective of average serum T concentrations over 24 hours (Cavg0-24).
Design: This was a double-blinded, placebo-controlled randomized trial.
Setting: The study was conducted at five academic centers.
Participants: Forty-seven symptomatic men 65 years old or older with an average of two morning T concentration less than 275 ng/dL participated in the study.
Intervention(s): Transdermal T or placebo gel was applied for 120 ± 14 days. Monthly dose adjustments were made if necessary to target serum T between 400 and 500 to 800 ng/dL.
Main outcome measures: Variability of serum T 2 hours after the gel application on two outpatient visits and at multiple time points over 24 hours during the inpatient day was measured.
Results: On-treatment T levels varied substantially on the 2 ambulatory days and over 24 hours during the inpatient day. Ambulatory 2-hour postapplication T levels did not correlate significantly with either 2-hour postapplication serum T or Cavg0-24 measured during the inpatient day. Only 22.2% of men receiving T had a Cavg0-24 within the target range of 500-800 ng/dL; 81.5% had a Cavg0-24 within the broader 300-1000 ng/dL range.
Conclusion: Large within-individual variations in serum T after T gel application render ambulatory 2-hour postapplication T level a poor indicator of average serum T on another day. Our data point out the limitations of dose adjustments based on a single postapplication serum T measurement.