Concave fingernails are fairly uncommon, but can indicate a possible metabolic or other health concern. An iron deficiency can result in concave fingernails in addition to raised ridges. The medical term for this condition is Koilonychia. Other causes may include coronary disease or hypothyroidism. In infants this condition may be present for the first few years of life, after which it may clear on its own. Consultation with a qualified physician can lead to the underlying cause and possible treatment.