I noticed a single gray line, appearing on my thumb nail, which goes from the tip of the nail to end.vertically.could this be a sign of a deficiency?

Probably benign: The diagnosis that seems to most closely fit with your description (without a picture) is melanonychia striata, which is most common in the thumb. It results from increased melanin deposit in the nail plate. This can be caused by bleeding into the nail plate, a small foreign body, fungal infection or wart, or tumor-like growth of melanocyte. A skilled dermatologist needs to examine it for you.