Testosterone is the male sex hormone responsible for male muscle mass, hair growth, libido, bone health, and RBC production.  Testosterone not only drives sex differentiation and development but is also key in sustaining and promoting spermatogenesis.  Low levels of testosterone, although not necessarily predictive of spermatogenic failure, can potentiate poor sperm production and low sperm concentrations.  Testosterone levels peak early in the morning and AM levels are preferred for evaluation purposes.  In post-cancer treatment, the best predictors of low testosterone is associated with patient age and residual testicular volume < 12cc [5].

It is interesting to note that half of testicular cancer survivors will have at least one abnormality long term in testosterone, LH, or FSH following treatment [2].