What Causes Brittle Nails?

Brittle nails can be a normal result of aging. Or they can indicate a medical abnormality.

Here is a list of things that can cause your nails to be brittle, weak and break easily, courtesy of the U.S. National Library of Medicine:
Frequent use of nail polish, remover, hardeners or other cosmetic nail applications.
Reduced amounts of zinc or iron in the body.
Problems with the thyroid.