Re: what causes thumb nail indentations?
Google the Mayo Clinic on nail conditions. They have a short slide show of 7 medical conditions and photos.
Be careful with trying to diagnose. You must very specifically identify the types of lines.
For example, my thumbnails are wavy and not sharp horizontal lines so they are not Beau's lines. Your's may be. I do not take iron supplements or drink tap water (minerals), which supports a probable lack of iron, so it could be true. I finally found this....
Koilonychia (WAVY Nails): Nails may exhibit many different abnormalities. In the condition known as koilonychia, the nails are flattened and have concavities. This condition may be associated with iron deficiency.
Again - you may have Beau's lines. In any case, use online articles with photos to identify what you have.