Gonna have to agree with RickV. You are the man in the relationship - you don't need her permission.
That said, unless you have a legal prescription for TRT - then you do have to consider legality. Granted, chances of getting in trouble for gear used for personal use is extremely slim, but something to consider.
It also depends on how old you are. If you're in your mid to late 30's or older, then you could get a test to see how your testosterone levels are doing.
If that isn't an option, then you have 1 of 2 options:
1. Just take them and if she finds out - apologize for not telling her about using them, but don't apologize for actually using them. Don't use the words "I'm sorry."
2. Tell her you're going to start using testosterone for health and fitness reasons - and that you'll get blood work done before, during, and after - to help ease her mind about it. But state it as something that you are going to do. You don't ask her for permission.