What does a single grey line on the nail of thumb means?

NailsAre LikeSnails: Depends on whether it's parallel or perpendicular to the nail's long axis (love that jargon!)In general, it can be a sign of recent trauma or evidence of a past illness(if it were in the middle, that illness could have been a few months ago).Some medical problems(kidney, liver)can lead to diminished protein in the blood and body and can do it to but you'd probably have other symptoms too.Picture?

Need more info: Horizontal or vertical line? One nail or multiple nails with the same patter? If it is on a single nail, horizontal or vertical, it does not mean anything. If all nails have a horizontal line, that may represent exposure to some toxin. Arsenic produces such lines.

Derm r/o melanoma: Probably benign melanonychia, but best show it to your doctor or dermatologist just to make sure it is not melanoma, which prefers single thumb and great toenail occurence. http://www.dermnetnz.org/doctors/dermoscopy-course/nails.html