Diurnal rhythms of serum total, free and bioavailable testosterone and of SHBG in middle-aged men compared with those in young men.

Background: Conflicting views are reported on the association between advancing age and gradually diminishing concentrations of serum total testosterone in men. The putative loss of diurnal rhythm in serum total testosterone in older men is reported to be in part due to low concentrations in the morning when compared to concentrations found in young men. We have measured total, free and bioavailable testosterone along with SHBG in samples taken every 30 min throughout a 24-h period in 10 young and eight middle-aged men.