Hypogonadism and Testosterone Assays

Normal testosterone (TTT) in an assay at the BCCA laboratory is now defined as >3.5ug/ml. It combines free TTT (left) with albumin bound TTT (top right) and sex hormone binding globulin bound TTT (bottom right) – figure 3. These make up approximately 1-2%, 35% and 65% of TTT respectively. The first two combined make up the bioavailable TTT. This is the important component in a ‘true’ diagnosis of hypogonadism, but is not measured by many laboratories. It is therefore possible for a man to have a ‘low normal’ TTT at BCCA and have hypogonadism defined by the bioTTT. Testosterone levels decline with age (figure 4).

insert fig. 3

Figure 3: TTT free and bound - see text.

insert fig.4

Figure 4: increasing incidence of hypogonadism with age by various assays. Harman et al. J Clin endocrinal Metab 2001; 86(2): 724-31.