I have this problem too, and I know I am low in B12, D, and iron, probably other vitamins also that I am unaware of. I also am hypothyroid, and I have found a lot of people with thyroid trouble have nail problems. I don’t know how it is all connected, but I believe it is. For a while I was getting horizontal white marks across my nails also, but that seems to have resolved itself now that I have been taking more vitamins and minerals. They do sell vitamins specifically for nails, I think they contain Biotin. I thought they promote them for nail strength? Not sure if it works for ridges also?