I have read that injury can cause the nails to be permanently thick and yellow, and fall off. Your doc is probably right, there might not be anything you can do.

However, just in case this rings a bell with you, this is my story:

I had yellow nails for years, tried every antifungal remedy and nothing helped, except using an exfoliating foot cream on my feet. Then over the last two years one becamse thick and crusty. When I discovered I had gout in one knee, I changed my diet for much less meat, powdered cherry supplements, etc. to reduce gout, and after two months I noticed there was a straight line across my toenail where the bottom was completely normal, and the crusty yellow was growing out. It's almost gone now at nearly 5 months.

Hope that helps you or someone else out there, it may or may not be what you have.