Removal of the testes greatly reduces any chance of testicular cancer. Testosterone is not stored by the body for future use, so in order to maintain healthy levels, it must be administered in timed intervals and in appropriate dosages. Testosterone can worsen or cause polycythemia by stimulating bone marrow to produce more red blood cells. If he no longer has functional ovaries due to oophorectomy or some other reason, then the cessation of testosterone would leave him without a significant amount of either testosterone or estrogen in his system.
Tell your physician about all your health history. If things go south, find another provider. Emotional changes. Check out the following two web sites, which contain free, downloadable information written by care providers for care providers and trans patients.