Measurement of testosterone: how important is a morning blood draw?

AbstractObjective:Since testosterone levels exhibit a circadian variation with peak levels in the morning, evidence-based guidelines recommend measuring morning total testosterone (TT) levels as the initial diagnostic test for androgen deficiency. However, it has been suggested that morning blood draw may not be necessary in older men due to a blunted circadian rhythm. We sought to determine whether it is possible to expand the morning sampling window for measurement of TT.Research Design and Methods:TT levels were measured in a subset of men (mean age of 61 years) participating in the 2013 Prostate Cancer Awareness Week.Results:TT levels measured in blood drawn from 8 to 11 AM (n = 229) differed significantly from those drawn outside this window (n = 442) (411.7 vs 368.3 ng/dl; p = 0.0003). Differences in TT levels were evident across five blood draw time windows (p = < 0.0001) and persisted after adjustment for age and BMI. TT levels in blood drawn from 2 to 5 PM (344.3 ng/dl) and 5 to 8 PM (334.4 ng/dl...