White fingernails with pink near the tips can indicate cirrhosis of the liver; if the nail is completely white, there could be a kidney or liver disorder, or severe anemia.
Yellow fingernails can mean that you are diabetic, have problems with your liver, your lymphatic system, or respiratory disorders. Signs of these disorders can show up in the nails years before they actually occur, though, so just because your nails may be yellow doesn’t necessarily mean that you are currently suffering from such diseases. They’re just things to look out for, so talk to your doctor and take the proper preventative measures.
Dark nails, which will usually be flat or thin, indicate a deficiency in vitamin B-12.
If your nails beds are a very deep blue, it could mean emphysema or pulmonary obstruction.
White lines across the nail can indicate liver disease.
If your nail is half white, with dark spots across the top, there may be a problem with your kidneys.
Spots of pitted brown, or nail tips that split easily, can indicate psoriasis.
Very red skin at the bottom of your nail bed could mean a connective tissue disorder.
Thyroid disorders may cause the nails to have ridges or bumps, separate from the nail bed, or to be unusually soft.
Highly bendable nails can be an indicator of rheumatoid arthritis.
Flat nails can indicate Raynaud’s disease, a disease of the circulatory system that leaves the hands and feet perpetually cold.
Abnormally wide, square nails may signal a hormonal disorder.
Spoon-shaped nails indicate an iron deficiency or difficulty with the thyroid.
Abnormally large lunula (the white half-moon shape at the base of the nail) suggests an overactive thyroid, while the absence of a lunula can mean an under-active thyroid.
You don’t have to go rushing to your doctor for a few white spots or white lines on your nails; those marks (scientific name: leuconychia) are indicative of trauma to the nail. You may have bumped it and not even noticed, and a white spot appearing in that area is perfectly normal. Such spot and lines can also be caused if, when the nail is forming, the cuticle is pushed back too roughly. (The white areas will grow out normally with the nail and can be clipped off once they reach the end.) You’ll only need to see your doctor if your nails look very strange to you, a lot different than usual, or if changes happen very suddenly.