The causes of fingernail ridges vary from common and harmless to rare and threatening. If you have noticed formation of ridges in your fingernails, the likely cause depends on the particular nature of those ridges.

Normal Vertical Ridges

Ridges extending from the base to the tip of the fingernail are termed vertical; they are often the result of heredity, of your body's natural aging, or of the conditions in which you use your hands. Nails grow out from under the base of the cuticle along the nail bed. According to Mayo Clinic staffers, as you get older the natural decrease in oils and moisture in the nails makes normal grooves in the underlying nail bed more apparent. Prolonged immersion in water or exposure to harsh cleaning products will also contribute to this dryness. You can often improve the condition by applying a good-quality nail and cuticle product twice a day.

Problem-Related Vertical Ridges

Some vertical ridges in fingernails are possible indicators of serious medical conditions. These include hypothyroidism, the overall bodily slowdown that results from an under-active thyroid gland. As the hypothyroid person's sweat glands produce less moisture, their nails and nail beds dry out, become brittle, and develop vertical ridges. Some rheumatoid arthritis patients also report vertical fingernail ridges in conjunction with the disease. Vitamin or mineral deficiencies may also contribute to vertical ridges; for instance, iron deficiency anemia may cause vertically-ridged nails that are also dished inward (concave) and thin.

Horizontal Fingernail Ridges

Horizontal ridges in the fingernails (often called ‘Beau's lines') are depressions, sometimes with a darkened line, that extend from one side of the nail to the other. They signal that the formation of the nail plate was temporarily halted. Causes for this interruption range from malnutrition to severe illness, chemotherapy or a trauma to one or more fingernails. In rare cases, horizontal ridges can be associated with arsenic poisoning. Because they are linked to an array of health issues, a visit to your physician is a good idea if your nails begin to form persistent horizontal ridges.