Testosterone is a steroid and steroids are not particular soluble in blood. In order for steroids to travel easily around the body they bind to a protein called sex hormone binding globulin. Only about 1-2% of the total testosterone circulating in the blood stream at any one time is free (unbounded) from that transport protein. So, if one wants to know how well ADT is doing to suppress testosterone and control the cancer, should they measure the total testosterone or the free testosterone? The authors make the case that free testosterone is a more reliable measure than is total testosterone in terms of cancer control.
Regis L, Planas J, Carles J, Maldonado X, Comas I, Ferrer R, Morote J. 2016. Free testosterone during androgen deprivation therapy predicts castration-resistant progression better than total testosterone. Prostate 77(1):114-120. www.ncbi.nlm.nih.gov/pubmed/27800640