Before HRT, I had very strong nails. After about a year into HRT, I they were brittle and breaking when much longer then the fingertip. I really wasn't too concerned at that time, there were much bigger issues then growing a sexy set of nails..... However, I was concerned about my hair, and started taking Biotin (5000 mcg, a dose usually recommended for vegetarians), and noticed a side effect of stronger nails, I think!

With all of the changes going on with HRT, it's really hard to tell what's doing what, and what changes over time. That was 1.5 years or so, ago.
At this point, I'm still on Biotin, growing nails well, and have no idea if Biotin's the thing that's making my hair a bit better, or just experience with caring for it. In regards to nails, they are still growing strong, and I've recently adopted the use of Sally Hansen's witchcraft, the 4-in-one, 'which' is utterly amazing for getting even longer, talon-like growth! Meow! I started using that about two months ago.....
Good luck!
Melan