In the end, it is only going to grow and grow and cause more problems.
It is already 16mm and while you did not give information on where it sits (some grow up into the optic are, some wrap around blood vessels, some down into the cavernous sinus) it will eventually hit an area where it will cause either a critical issue like blindness or stroke, it can explode, or the action of the hormones going off can cause problems. It can make your quality of life progressively worse and yes, then it can kill you.
I am on replacements myself - it is pills, patches, and shots to be frank. It is not an easy life but it is actually to me a lot better than I was before.
It will not stop growing on its own. You may also need radiation to kill areas that are inoperable - again, I don't know where yours tumor sits.
I understand your fear completely!!! But you have to have this one out. You need a center of excellence and a very experienced surgeon (do not settle here!) and with that you may retain some hormones.
Please post back with questions or comments... before or after surgery.
Where in your brain is the tumor?