I was wondering about this too. Thanks for giving us some answers.
Thank you for your answer :)
Horizontal ridges and white lines on nails are seen in trauma to nail bed, kidney diseases, pellagra or Vit B3 deficiency, low protein, arsenic poisoning, and liver diseases. Hence a kidney and liver function test should be done.
http://www.mayoclinic.com/health/nails/WO00055
all i can say is either go to your doc if your concerned or check out this web page !
best of luck !