Fingernails can grow downward as a result of a condition called nail clubbing in which the fingers grow larger, while the nails grow around and curve over the larger fingertips, explains Mayo Clinic. Health problems such as low blood oxygen, inflammatory bowel disease, AIDS and cardiovascular disease can cause clubbing.
In some cases, nail clubbing runs in families and is not associated with any health problems, notes NHS Choices. However, when a person suddenly develops nail clubbing, it can be a warning sign of diseases of the lungs and heart, such as lung cancer, endocarditis and bronchiectasis. Other health issues that nail clubbing may indicate include cirrhosis, bowel cancer, stomach cancer and polycythaemia, a condition in which the blood is thicker than normal.
Some other fingernail and toenail abnormalities that can provide clues about health are brittle and crumbly nails, loose nails, thickened nails and discolored nails, according to NHS Choices. Possible causes of brittle, crumbly nails include fungal infections, thyroid problems and the skin condition lichen planus. Loose nails can signal overactive thyroid, poor circulation, an allergic reaction to a medication or a fungal infection. Potential reasons for thickened, overgrown nails include fungal infections, reactive arthritis, or shoes that are too narrow or small. Nails that are discolored a yellowish tint may result from sinusitis, jaundice, lymphoedema or tuberculosis.