Evidence suggests that even when advanced prostate cancer becomes resistant to androgen deprivation therapy (Lupron or Zoladex), it is useful to continue on androgen deprivation because excess testosterone still allows for cancer cells to progress faster and spread throughout the body.
van der Sluis TM, Bijnsdorp IV, Jacobs JJ, Meuleman EJ, Rozendaal L, Geldof AA, van Moorselaar RJ, Vis AN. Serum testosterone plays an important role in the metastatic ability of castration resistant prostate cancer. World Journal of Urology. 2012 Oct 19.